Prevent base images from accidental prune

Base images sometimes used to be pruned as part of a rebuild when they
became dangling.  That could happen when the "rpm" image was based on a
different version of the base image.

This was super annoying since you had to wait for the whole package set
to be downloaded and installed again.

Luckily, base images don't change as often so we can perhaps afford to
keep each version around.  Do that by simply adding the Dockerfile hash
to the tag name.

(cherry picked from commit 766192a)

tests/mktree.oci

@@ -28,7 +28,7 @@ fi
 CACHE_DIR="mktree.cache"
 IID_FILE="$CACHE_DIR/image-id"
 IMAGE_ID=$(cat $IID_FILE 2>/dev/null || echo "")
-BASE_TAG="rpm/base"
+BASE_TAG="rpm/base:$(sha256sum Dockerfile | head -c8)"
 ARGS="-f Dockerfile $FROM $CONTEXT"
 ROOTLESS=$([ $(id -u) == 0 ] && echo 0 || echo 1)
 CMD=$1; shift