cmd-build-with-buildah: add --direct

Support running buildah directly rather than through supermin. This
is useful for devs like me who have buildah actually just proxy out of
their container and to the host buildah.

Author: jlebon

src/cmd-build-with-buildah

@@ -15,13 +15,15 @@ Usage: coreos-assembler build-with-buildah
 
   The following options are supported:
   --versionary          Use the versionary script from the source config to drive version.
+  --direct              Run buildah directly rather than within supermin.
 EOF
 }
 
 VERSIONARY=
 VERSION=
+DIRECT=
 rc=0
-options=$(getopt --options h,v --longoptions help,versionary -- "$@") || rc=$?
+options=$(getopt --options h,v,d --longoptions help,versionary,direct -- "$@") || rc=$?
 [ $rc -eq 0 ] || {
     print_help
     exit 1
@@ -36,6 +38,9 @@ while true; do
         -v | --versionary)
             VERSIONARY=1
             ;;
+        -d | --direct)
+            DIRECT=1
+            ;;
         --)
             shift
             break
@@ -80,8 +85,14 @@ build_with_buildah() {
         set -- "$@" --build-arg VERSION="${VERSION}"
     fi
 
-    /usr/lib/coreos-assembler/cmd-supermin-run --cache \
-        env -C "${tempdir}/src" TMPDIR="$(realpath cache)" buildah "$@" .
+    if [ -n "$DIRECT" ]; then
+        # turn on layer caching in the direct case; it wouldn't hurt in the
+        # supermin path, but it'd be a waste of space on the rootfs
+        env -C "${tempdir}/src" buildah "$@" --layers=true .
+    else
+        /usr/lib/coreos-assembler/cmd-supermin-run --cache \
+            env -C "${tempdir}/src" TMPDIR="$(realpath cache)" buildah "$@" .
+    fi
 
     /usr/lib/coreos-assembler/cmd-import "oci-archive:${tmp_oci_archive_path}"