[mkimage] Clean hostname and resolv.conf

These depend on the build environment, but differences
don't show up inside the docker image as docker sets the
content of these files when starting a container.

We can put static values in the files to stop the image
layers differing without affecting the running containers
in any way.

This wasn't caught by the reproducibility check as the values
will be pretty static when built on the same host. (hostname
will be fixed, and resolv.conf will only change if the networking
configration changes enough, and that is unlikely to happen)

mkimage

@@ -203,10 +203,17 @@ chmod 0755 "$rootfsDir/usr/sbin/install_packages"
 # We also have to copy over the backup at /etc/shadow- so that it doesn't change
 chroot "$rootfsDir" getent passwd | cut -d: -f1 | xargs -n 1 chroot "$rootfsDir" chage -d 17885 && cp "$rootfsDir/etc/shadow" "$rootfsDir/etc/shadow-"
 
+# Clean /etc/hostname and /etc/resolv.conf as they are based on the current env, so make
+# the chroot different. Docker doesn't care about them, as it fills them when starting
+# a container
+echo "" > "$rootfsDir/etc/resolv.conf"
+echo "host" > "$rootfsDir/etc/hostname"
+
 # Capture the most recent date that a package in the image was changed.
 # We don't care about the particular date, or which package it comes from,
 # we just need a date that isn't very far in the past.
 BUILD_DATE="$(find $rootfsDir/usr/share/doc -name changelog.Debian.gz -exec dpkg-parsechangelog -SDate -l'{}' \; | xargs -l -i date --date="{}" +%s | sort -n | tail -n 1)"
+
 echo "Trimming down"
 for DIR in $DIRS_TO_TRIM; do
   rm -r "$rootfsDir/$DIR"/*