build: revamp make targets and update build instructions

- Replace `make build_linux_amd64` and `make build_linux_arm64` with `make build_docker`
- Add a help target with usage instructions and target descriptions
- Update `fmt` to install the latest version of `gofumpt`
- Add descriptions for `vet`, `fmt-check`, `test`, `install`, `build`, `ssh-server`, and `clean` targets
- Remove several build targets (`amd64`, `i386`, `arm64`, `arm`)
- Remove the `coverage` and `version` targets

Signed-off-by: appleboy <appleboy.tw@gmail.com>

.github/workflows/docker.yml

@@ -25,8 +25,7 @@ jobs:
 
       - name: Build binary
         run: |
-          make build_linux_amd64
-          make build_linux_arm64
+          make build_docker
 
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v3