[CI-23743]: Support loading hosted certs in windows (#518)

The plugin now reads a HARNESS_CA_PATH environment variable pointing to a PEM CA file and installs it into the host system trust store before starting the Docker daemon. This solves x509 errors when builds run behind TLS-intercepting egress proxies that re-sign upstream TLS with their own CA.

- Linux: installs into distro anchor directory (/usr/local/share/ca-certificates or /etc/pki/ca-trust/source/anchors) and refreshes system bundle via update-ca-certificates/update-ca-trust, with direct bundle append fallback
- Windows: imports via certutil -addstore Root
- macOS: logged no-op (not currently supported)
- Best-effort, idempotent, documented in README.md with usage examples
- New platform-specific source files: ca_linux.go, ca_windows.go, ca_other.go
- Comprehensive tests added: ca_test.go, ca_linux_test.go

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

AI-Session-Id: fe2aa5fe-1bf5-4257-ab76-2c862c5637fe
AI-Tool: claude-code
AI-Model: unknown

.harness/harness.yaml

@@ -804,6 +804,74 @@ pipeline:
                             - acr
               buildIntelligence:
                 enabled: false
+        - stage:
+            name: win-ltsc-2025-amd64
+            identifier: winltsc2025amd64
+            description: ""
+            type: CI
+            spec:
+              cloneCodebase: true
+              caching:
+                enabled: false
+                override: false
+                paths: []
+              buildIntelligence:
+                enabled: false
+              infrastructure:
+                type: VM
+                spec:
+                  type: Pool
+                  spec:
+                    poolName: windows-2025
+                    os: Windows
+              execution:
+                steps:
+                  - step:
+                      type: Run
+                      name: Build Binary 2025
+                      identifier: Build_Binary_2025
+                      spec:
+                        connectorRef: Plugins_Docker_Hub_Connector
+                        image: golang:1.23.0
+                        shell: Sh
+                        command: |-
+                          # disable cgo
+                          export CGO_ENABLED=0
+
+                          go build -o release/windows/amd64/drone-<+matrix.repo>.exe ./cmd/drone-<+matrix.repo>
+                      strategy:
+                        matrix:
+                          repo:
+                            - docker
+                            - gcr
+                            - gar
+                            - ecr
+                            - acr
+                  - step:
+                      type: Plugin
+                      name: Plugin_1
+                      identifier: Plugin_1
+                      spec:
+                        connectorRef: Plugins_Docker_Hub_Connector
+                        image: plugins/docker:windows-ltsc2025-amd64
+                        settings:
+                          username: drone
+                          password: <+secrets.getValue("Plugins_Docker_Hub_Pat")>
+                          repo: plugins/<+matrix.repo>
+                          dockerfile: docker/<+matrix.repo>/Dockerfile.windows.amd64.ltsc2025
+                          auto_tag: "true"
+                          auto_tag_suffix: windows-ltsc2025-amd64
+                      strategy:
+                        matrix:
+                          repo:
+                            - docker
+                            - gcr
+                            - gar
+                            - ecr
+                            - acr
+                      when:
+                        stageStatus: Success
+                        condition: <+codebase.build.type> == "tag"
     - stage:
         name: Manifest and Release
         identifier: Manifest

README.md

@@ -114,6 +114,54 @@ COOL BANANAS
 ```
 
 
+### Trusting a custom / egress-proxy CA (`HARNESS_CA_PATH`)
+
+When builds run behind a TLS-intercepting egress proxy (e.g. Harness egress
+control), the proxy re-signs upstream TLS with its own CA. The Docker daemon and
+its embedded BuildKit verify registry TLS against the **host system trust
+store**, so base-image pulls fail with
+`x509: certificate signed by unknown authority` unless that CA is trusted.
+
+Set the `HARNESS_CA_PATH` environment variable to a PEM CA (bundle) file and the
+plugin installs it into the host trust store **before starting the Docker
+daemon**:
+
+```yaml
+steps:
+- name: build and push
+  image: plugins/docker
+  pull: never
+  environment:
+    HARNESS_CA_PATH: /etc/harness-certs/ca.crt
+  settings:
+    repo: octocat/hello-world
+    tags: latest
+```
+
+Behavior:
+
+- **Linux** — installs into the distro anchor dir
+  (`/usr/local/share/ca-certificates` on Debian/Ubuntu/Alpine,
+  `/etc/pki/ca-trust/source/anchors` on RHEL/CentOS/Fedora) and runs
+  `update-ca-certificates` / `update-ca-trust`. It also appends the CA directly
+  to the consolidated bundle (`/etc/ssl/certs/ca-certificates.crt` or the RHEL
+  equivalent) so trust holds even on minimal images without a refresh tool.
+- **Windows** — imports the CA into the machine trust store via
+  `certutil -addstore -f Root`.
+- **macOS / other** — logged no-op (not currently supported).
+
+Notes:
+
+- It is **best-effort and idempotent**: when `HARNESS_CA_PATH` is unset, the file
+  is missing, or the file is empty, the plugin logs and continues. Re-running the
+  step will not duplicate the CA in the bundle.
+- The CA is trusted for the daemon's own registry TLS (base-image pulls, cache
+  endpoints). To make the CA available to `RUN` steps inside the build, also pass
+  it as needed via the Dockerfile / build args.
+- This is separate from the proxy build args
+  (`http_proxy`/`https_proxy`/`no_proxy`), which the plugin already injects from
+  the environment (including `HARNESS_`-prefixed variants).
+
 ### Running from the CLI
 
 ```console

ca_linux.go

@@ -0,0 +1,110 @@
+//go:build linux
+// +build linux
+
+package docker
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+)
+
+// anchorCandidate is a distro CA anchor directory plus the tool that refreshes
+// the consolidated bundle from it.
+type anchorCandidate struct {
+	dir     string
+	refresh []string
+}
+
+// linuxAnchorCandidates and linuxBundleCandidates are package vars (not consts)
+// so tests can point them at a temp trust store. Order matters: the first
+// existing entry wins for the anchor install.
+var linuxAnchorCandidates = []anchorCandidate{
+	{"/usr/local/share/ca-certificates", []string{"update-ca-certificates"}},     // Debian/Ubuntu/Alpine
+	{"/etc/pki/ca-trust/source/anchors", []string{"update-ca-trust", "extract"}}, // RHEL/CentOS/Fedora
+}
+
+var linuxBundleCandidates = []string{
+	"/etc/ssl/certs/ca-certificates.crt", // Debian/Ubuntu/Alpine
+	"/etc/pki/tls/certs/ca-bundle.crt",   // RHEL/CentOS
+}
+
+// installHarnessCA drops the CA into the distro anchor directory and refreshes
+// the system bundle. It also appends directly to the consolidated bundle so
+// trust is effective even on images that lack update-ca-certificates (e.g.
+// minimal distros); the Docker daemon and Go's crypto/x509 read from that
+// bundle. Best-effort: failures are logged, never fatal.
+func installHarnessCA(caPEM []byte) {
+	anchorCandidates := linuxAnchorCandidates
+
+	installedViaAnchor := false
+	for _, c := range anchorCandidates {
+		if _, err := os.Stat(c.dir); err != nil {
+			continue
+		}
+		dst := filepath.Join(c.dir, "harness-egress-ca.crt")
+		if err := os.WriteFile(dst, caPEM, 0644); err != nil {
+			fmt.Printf("Could not write Harness CA to %s: %s\n", dst, err)
+			continue
+		}
+		if _, err := exec.LookPath(c.refresh[0]); err != nil {
+			continue
+		}
+		cmd := exec.Command(c.refresh[0], c.refresh[1:]...)
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+		if err := cmd.Run(); err != nil {
+			fmt.Printf("Warning: %s failed: %s\n", c.refresh[0], err)
+			continue
+		}
+		fmt.Printf("Installed Harness egress CA into %s and refreshed system trust store.\n", c.dir)
+		installedViaAnchor = true
+		break
+	}
+
+	// Belt-and-suspenders: append to the consolidated bundle(s) directly so the
+	// CA is trusted even when no refresh tool exists or the anchor step failed.
+	bundleCandidates := linuxBundleCandidates
+	appended := false
+	for _, bundle := range bundleCandidates {
+		if _, err := os.Stat(bundle); err != nil {
+			continue
+		}
+		if bundleContainsCA(bundle, caPEM) {
+			appended = true
+			continue
+		}
+		f, err := os.OpenFile(bundle, os.O_APPEND|os.O_WRONLY, 0644)
+		if err != nil {
+			fmt.Printf("Could not append Harness CA to %s: %s\n", bundle, err)
+			continue
+		}
+		payload := caPEM
+		if !bytes.HasPrefix(payload, []byte("\n")) {
+			payload = append([]byte("\n"), payload...)
+		}
+		if _, err := f.Write(payload); err != nil {
+			fmt.Printf("Could not append Harness CA to %s: %s\n", bundle, err)
+		} else {
+			fmt.Printf("Appended Harness egress CA to %s.\n", bundle)
+			appended = true
+		}
+		f.Close()
+	}
+
+	if !installedViaAnchor && !appended {
+		fmt.Println("Warning: could not install the Harness egress CA into any known trust store location; base-image pulls through the egress proxy may fail with x509 errors.")
+	}
+}
+
+// bundleContainsCA reports whether the CA bytes are already present in the
+// bundle, to keep the step idempotent across retries.
+func bundleContainsCA(bundle string, caPEM []byte) bool {
+	existing, err := os.ReadFile(bundle)
+	if err != nil {
+		return false
+	}
+	return bytes.Contains(existing, bytes.TrimSpace(caPEM))
+}

ca_linux_test.go

@@ -0,0 +1,82 @@
+//go:build linux
+// +build linux
+
+package docker
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// withTempTrustStore points the Linux anchor/bundle candidates at a temp dir so
+// installHarnessCA can be exercised without touching the real system store.
+func withTempTrustStore(t *testing.T) (anchorDir, bundlePath string) {
+	t.Helper()
+	dir := t.TempDir()
+	anchorDir = filepath.Join(dir, "anchors")
+	bundlePath = filepath.Join(dir, "ca-certificates.crt")
+	if err := os.MkdirAll(anchorDir, 0755); err != nil {
+		t.Fatal(err)
+	}
+	// Seed an existing bundle so the append path is exercised.
+	if err := os.WriteFile(bundlePath, []byte("# existing roots\n"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	origAnchors := linuxAnchorCandidates
+	origBundles := linuxBundleCandidates
+	// No refresh tool on the anchor entry so the test doesn't shell out.
+	linuxAnchorCandidates = []anchorCandidate{{dir: anchorDir, refresh: []string{"harness-nonexistent-refresh-tool"}}}
+	linuxBundleCandidates = []string{bundlePath}
+	t.Cleanup(func() {
+		linuxAnchorCandidates = origAnchors
+		linuxBundleCandidates = origBundles
+	})
+	return anchorDir, bundlePath
+}
+
+func TestInstallHarnessCA_Linux(t *testing.T) {
+	anchorDir, bundlePath := withTempTrustStore(t)
+
+	installHarnessCA([]byte(testCAPEM))
+
+	// Anchor file should be written.
+	anchorFile := filepath.Join(anchorDir, "harness-egress-ca.crt")
+	got, err := os.ReadFile(anchorFile)
+	if err != nil {
+		t.Fatalf("expected anchor file at %s: %s", anchorFile, err)
+	}
+	if string(got) != testCAPEM {
+		t.Fatalf("anchor file = %q, want %q", string(got), testCAPEM)
+	}
+
+	// Bundle should have the CA appended (refresh tool is absent, so the
+	// direct-append fallback is what provides trust here).
+	bundle, err := os.ReadFile(bundlePath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(string(bundle), strings.TrimSpace(testCAPEM)) {
+		t.Fatalf("bundle %q does not contain appended CA", string(bundle))
+	}
+	if !strings.HasPrefix(string(bundle), "# existing roots") {
+		t.Fatalf("bundle append clobbered existing roots: %q", string(bundle))
+	}
+}
+
+func TestInstallHarnessCA_Linux_Idempotent(t *testing.T) {
+	_, bundlePath := withTempTrustStore(t)
+
+	installHarnessCA([]byte(testCAPEM))
+	installHarnessCA([]byte(testCAPEM))
+
+	bundle, err := os.ReadFile(bundlePath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n := strings.Count(string(bundle), strings.TrimSpace(testCAPEM)); n != 1 {
+		t.Fatalf("expected CA to appear exactly once after two installs, got %d", n)
+	}
+}

ca_other.go

@@ -0,0 +1,12 @@
+//go:build !linux && !windows
+// +build !linux,!windows
+
+package docker
+
+import "fmt"
+
+// installHarnessCA is a no-op on platforms where egress control is not yet
+// supported (e.g. macOS). HARNESS_CA_PATH is honored on Linux and Windows.
+func installHarnessCA(caPEM []byte) {
+	fmt.Println("HARNESS_CA_PATH is set but egress-proxy CA trust injection is not supported on this platform; skipping.")
+}

ca_pem.go

@@ -0,0 +1,29 @@
+package docker
+
+import (
+	"encoding/pem"
+	"fmt"
+)
+
+// decodePEMCertificates extracts DER-encoded CERTIFICATE blocks from a PEM
+// (or PEM bundle). Non-certificate blocks are skipped. Returns an error when
+// no certificate blocks are present.
+func decodePEMCertificates(pemBytes []byte) ([][]byte, error) {
+	var ders [][]byte
+	rest := pemBytes
+	for {
+		var block *pem.Block
+		block, rest = pem.Decode(rest)
+		if block == nil {
+			break
+		}
+		if block.Type != "CERTIFICATE" {
+			continue
+		}
+		ders = append(ders, block.Bytes)
+	}
+	if len(ders) == 0 {
+		return nil, fmt.Errorf("no CERTIFICATE PEM blocks found")
+	}
+	return ders, nil
+}

ca_pem_test.go

@@ -0,0 +1,114 @@
+package docker
+
+import (
+	"bytes"
+	"crypto/x509"
+	"encoding/pem"
+	"testing"
+)
+
+// Real self-signed CAs generated for these tests (CN=HarnessTestCA / HarnessTestCA2).
+const harnessTestCAPEM = `-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
+`
+
+const harnessTestCA2PEM = `-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
+`
+
+func TestDecodePEMCertificates_Single(t *testing.T) {
+	ders, err := decodePEMCertificates([]byte(harnessTestCAPEM))
+	if err != nil {
+		t.Fatalf("decodePEMCertificates: %v", err)
+	}
+	if len(ders) != 1 {
+		t.Fatalf("got %d certs, want 1", len(ders))
+	}
+	cert, err := x509.ParseCertificate(ders[0])
+	if err != nil {
+		t.Fatalf("ParseCertificate: %v", err)
+	}
+	if cert.Subject.CommonName != "HarnessTestCA" {
+		t.Fatalf("CN = %q, want HarnessTestCA", cert.Subject.CommonName)
+	}
+}
+
+func TestDecodePEMCertificates_Bundle(t *testing.T) {
+	bundle := harnessTestCAPEM + "\n" + harnessTestCA2PEM
+	ders, err := decodePEMCertificates([]byte(bundle))
+	if err != nil {
+		t.Fatalf("decodePEMCertificates: %v", err)
+	}
+	if len(ders) != 2 {
+		t.Fatalf("got %d certs, want 2", len(ders))
+	}
+	for i, der := range ders {
+		if _, err := x509.ParseCertificate(der); err != nil {
+			t.Fatalf("cert %d: ParseCertificate: %v", i, err)
+		}
+	}
+}
+
+func TestDecodePEMCertificates_SkipsNonCertificateBlocks(t *testing.T) {
+	keyPEM := pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY", Bytes: []byte("not-a-key")})
+	mixed := append(append([]byte{}, keyPEM...), []byte(harnessTestCAPEM)...)
+	ders, err := decodePEMCertificates(mixed)
+	if err != nil {
+		t.Fatalf("decodePEMCertificates: %v", err)
+	}
+	if len(ders) != 1 {
+		t.Fatalf("got %d certs, want 1 (private key block skipped)", len(ders))
+	}
+}
+
+func TestDecodePEMCertificates_Empty(t *testing.T) {
+	_, err := decodePEMCertificates([]byte("not a pem\n"))
+	if err == nil {
+		t.Fatal("expected error for input with no CERTIFICATE blocks")
+	}
+}
+
+func TestDecodePEMCertificates_MatchesOpenSSLDER(t *testing.T) {
+	ders, err := decodePEMCertificates([]byte(harnessTestCAPEM))
+	if err != nil {
+		t.Fatal(err)
+	}
+	origBlock, _ := pem.Decode([]byte(harnessTestCAPEM))
+	if !bytes.Equal(origBlock.Bytes, ders[0]) {
+		t.Fatal("decoded DER does not match original PEM payload")
+	}
+}

ca_test.go

@@ -0,0 +1,87 @@
+package docker
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+const testCAPEM = `-----BEGIN CERTIFICATE-----
+MIIBAg==
+-----END CERTIFICATE-----
+`
+
+// withStubbedInstall replaces the platform installer with a capturing stub for
+// the duration of a test, returning a pointer to the captured CA bytes and a
+// pointer to a call counter.
+func withStubbedInstall(t *testing.T) (captured *[]byte, calls *int) {
+	t.Helper()
+	captured = new([]byte)
+	calls = new(int)
+	orig := installHarnessCAFn
+	installHarnessCAFn = func(caPEM []byte) {
+		*calls++
+		*captured = append([]byte(nil), caPEM...)
+	}
+	t.Cleanup(func() { installHarnessCAFn = orig })
+	return captured, calls
+}
+
+func TestTrustHarnessCA_Unset(t *testing.T) {
+	os.Unsetenv("HARNESS_CA_PATH")
+	_, calls := withStubbedInstall(t)
+
+	trustHarnessCA()
+
+	if *calls != 0 {
+		t.Fatalf("expected installer not to be called when HARNESS_CA_PATH is unset, got %d calls", *calls)
+	}
+}
+
+func TestTrustHarnessCA_MissingFile(t *testing.T) {
+	missing := filepath.Join(t.TempDir(), "does-not-exist.crt")
+	os.Setenv("HARNESS_CA_PATH", missing)
+	defer os.Unsetenv("HARNESS_CA_PATH")
+	_, calls := withStubbedInstall(t)
+
+	trustHarnessCA()
+
+	if *calls != 0 {
+		t.Fatalf("expected installer not to be called when CA file is missing, got %d calls", *calls)
+	}
+}
+
+func TestTrustHarnessCA_EmptyFile(t *testing.T) {
+	empty := filepath.Join(t.TempDir(), "empty.crt")
+	if err := os.WriteFile(empty, []byte("   \n\t"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	os.Setenv("HARNESS_CA_PATH", empty)
+	defer os.Unsetenv("HARNESS_CA_PATH")
+	_, calls := withStubbedInstall(t)
+
+	trustHarnessCA()
+
+	if *calls != 0 {
+		t.Fatalf("expected installer not to be called for whitespace-only CA, got %d calls", *calls)
+	}
+}
+
+func TestTrustHarnessCA_ValidFile(t *testing.T) {
+	valid := filepath.Join(t.TempDir(), "ca.crt")
+	if err := os.WriteFile(valid, []byte(testCAPEM), 0644); err != nil {
+		t.Fatal(err)
+	}
+	os.Setenv("HARNESS_CA_PATH", valid)
+	defer os.Unsetenv("HARNESS_CA_PATH")
+	captured, calls := withStubbedInstall(t)
+
+	trustHarnessCA()
+
+	if *calls != 1 {
+		t.Fatalf("expected installer to be called once for a valid CA, got %d calls", *calls)
+	}
+	if string(*captured) != testCAPEM {
+		t.Fatalf("installer received %q, want %q", string(*captured), testCAPEM)
+	}
+}

ca_windows.go

@@ -0,0 +1,83 @@
+//go:build windows
+// +build windows
+
+package docker
+
+import (
+	"fmt"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+// installHarnessCA imports the Harness egress-proxy CA into the Windows
+// certificate store under LocalMachine\Root (the machine-wide trusted roots the
+// Docker daemon and Go's crypto/x509 consult). Best-effort: failures are logged,
+// never fatal.
+//
+// Uses CryptoAPI (crypt32.dll) directly so it works on Nano Server images that
+// lack certutil and PowerShell. CERT_STORE_ADD_REPLACE_EXISTING keeps the step
+// idempotent across retries.
+func installHarnessCA(caPEM []byte) {
+	ders, err := decodePEMCertificates(caPEM)
+	if err != nil {
+		fmt.Printf("Could not decode Harness CA PEM: %s\n", err)
+		fmt.Println("Base-image pulls through the egress proxy may fail with x509 errors.")
+		return
+	}
+
+	storeName, err := windows.UTF16PtrFromString("ROOT")
+	if err != nil {
+		fmt.Printf("Could not prepare Windows root store name: %s\n", err)
+		return
+	}
+
+	store, err := windows.CertOpenStore(
+		windows.CERT_STORE_PROV_SYSTEM,
+		0,
+		0,
+		windows.CERT_SYSTEM_STORE_LOCAL_MACHINE,
+		uintptr(unsafe.Pointer(storeName)),
+	)
+	if err != nil {
+		fmt.Printf("Could not open LocalMachine\\Root store: %s\n", err)
+		fmt.Println("Base-image pulls through the egress proxy may fail with x509 errors.")
+		return
+	}
+	defer windows.CertCloseStore(store, 0)
+
+	for i, der := range ders {
+		if err := addCertToStore(store, der); err != nil {
+			fmt.Printf("Warning: failed to add Harness CA cert %d to LocalMachine\\Root: %s\n", i+1, err)
+			fmt.Println("Base-image pulls through the egress proxy may fail with x509 errors.")
+			return
+		}
+	}
+
+	fmt.Printf("Installed %d Harness egress CA certificate(s) into the Windows LocalMachine\\Root store.\n", len(ders))
+}
+
+func addCertToStore(store windows.Handle, der []byte) error {
+	if len(der) == 0 {
+		return fmt.Errorf("empty DER certificate")
+	}
+	ctx, err := windows.CertCreateCertificateContext(
+		windows.X509_ASN_ENCODING|windows.PKCS_7_ASN_ENCODING,
+		&der[0],
+		uint32(len(der)),
+	)
+	if err != nil {
+		return fmt.Errorf("CertCreateCertificateContext: %w", err)
+	}
+	defer windows.CertFreeCertificateContext(ctx)
+
+	if err := windows.CertAddCertificateContextToStore(
+		store,
+		ctx,
+		windows.CERT_STORE_ADD_REPLACE_EXISTING,
+		nil,
+	); err != nil {
+		return fmt.Errorf("CertAddCertificateContextToStore: %w", err)
+	}
+	return nil
+}

docker.go

@@ -1,6 +1,7 @@
 package docker
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"os"
@@ -131,6 +132,14 @@ type (
 
 // Exec executes the plugin step
 func (p Plugin) Exec() error {
+	// Trust the Harness egress-proxy CA (if HARNESS_CA_PATH is set) BEFORE the
+	// daemon starts. The Docker daemon and its embedded BuildKit verify registry
+	// TLS against the host system trust store; when traffic goes through the
+	// TLS-intercepting egress proxy, the presented cert is signed by the Harness
+	// CA and must be trusted here or base-image pulls fail with an x509
+	// "unknown authority" error.
+	trustHarnessCA()
+
 	// start the Docker daemon server
 	if !p.Daemon.Disabled {
 		p.startDaemon()
@@ -331,7 +340,7 @@ func (p Plugin) Exec() error {
 
 		if digest, err := getDigest(p.Build.TempTag); err == nil {
 			fmt.Printf("🔐 Found image digest: %s\n", digest)
-			
+
 			// Sign with digest reference
 			imageRef := fmt.Sprintf("%s@%s", p.Build.Repo, digest)
 			cosignCmd := createCosignCommand(imageRef, p.Cosign)
@@ -339,7 +348,7 @@ func (p Plugin) Exec() error {
 		} else {
 			fmt.Printf("⚠️  WARNING: Could not get image digest for cosign signing: %s\n", err)
 			fmt.Printf("   Falling back to tag-based signing\n")
-			
+
 			// Fall back to tag-based signing for each tag
 			for _, tag := range p.Build.Tags {
 				imageRef := fmt.Sprintf("%s:%s", p.Build.Repo, tag)
@@ -574,6 +583,35 @@ func getSecretCmdArg(kvp string, file bool) (string, error) {
 	return fmt.Sprintf("id=%s,env=%s", key, value), nil
 }
 
+// trustHarnessCA installs the Harness egress-proxy CA into the host trust store
+// so the Docker daemon / embedded BuildKit trust the TLS-intercepting proxy when
+// pulling base images. It is driven by HARNESS_CA_PATH (path to a PEM CA bundle)
+// and is a best-effort no-op when the var is unset, the file is missing, or the
+// platform isn't supported — build failures are surfaced later by docker itself.
+// The platform-specific installation is provided by installHarnessCA (see
+// ca_linux.go / ca_windows.go / ca_other.go); it is indirected through
+// installHarnessCAFn so tests can stub the actual system-trust write.
+var installHarnessCAFn = installHarnessCA
+
+func trustHarnessCA() {
+	caPath := os.Getenv("HARNESS_CA_PATH")
+	if caPath == "" {
+		return
+	}
+
+	caPEM, err := os.ReadFile(caPath)
+	if err != nil {
+		fmt.Printf("HARNESS_CA_PATH is set to %q but the CA could not be read: %s. Skipping CA trust.\n", caPath, err)
+		return
+	}
+	if len(bytes.TrimSpace(caPEM)) == 0 {
+		fmt.Printf("HARNESS_CA_PATH %q is empty. Skipping CA trust.\n", caPath)
+		return
+	}
+
+	installHarnessCAFn(caPEM)
+}
+
 // helper function to add proxy values from the environment
 func addProxyBuildArgs(build *Build) {
 	addProxyValue(build, "http_proxy")
@@ -853,7 +891,7 @@ func isValidPEMKey(pemContent string) bool {
 // createCosignCommand creates a cosign sign command with the given image reference
 func createCosignCommand(imageRef string, cosign CosignConfig) *exec.Cmd {
 	args := []string{"sign", "--yes"}
-	
+
 	// Handle private key (content vs file path)
 	if strings.HasPrefix(cosign.PrivateKey, "-----BEGIN") {
 		args = append(args, "--key", "env://COSIGN_PRIVATE_KEY")
@@ -861,21 +899,21 @@ func createCosignCommand(imageRef string, cosign CosignConfig) *exec.Cmd {
 	} else {
 		args = append(args, "--key", cosign.PrivateKey)
 	}
-	
+
 	// Set password if provided
 	if cosign.Password != "" {
 		os.Setenv("COSIGN_PASSWORD", cosign.Password)
 	}
-	
+
 	// Add any extra parameters
 	if cosign.Params != "" {
 		extraArgs := strings.Fields(cosign.Params)
 		args = append(args, extraArgs...)
 	}
-	
+
 	// Add the image reference to sign
 	args = append(args, imageRef)
-	
+
 	return exec.Command(cosignExe, args...)
 }
 

go.mod

@@ -17,6 +17,7 @@ require (
 	github.com/stretchr/testify v1.11.1
 	github.com/urfave/cli v1.22.2
 	golang.org/x/oauth2 v0.34.0
+	golang.org/x/sys v0.39.0
 	google.golang.org/api v0.187.0
 )
 
@@ -61,7 +62,6 @@ require (
 	go.opentelemetry.io/otel/trace v1.39.0 // indirect
 	golang.org/x/crypto v0.46.0 // indirect
 	golang.org/x/net v0.48.0 // indirect
-	golang.org/x/sys v0.39.0 // indirect
 	golang.org/x/text v0.32.0 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20251202230838-ff82c1b0f217 // indirect
 	google.golang.org/grpc v1.79.3 // indirect