cmdlib.sh: support manifest.yaml build_with_buildah key

We want to make sure that all of developers, CI and pipelines default to
the same buildah vs legacy path. Let's make the config repo itself drive
this.

Support a new `.metadata.build_with_buildah` key in `manifest.yaml`
which, if set to true, will make `cosa build` automatically use the
buildah path.

Note the `COSA_BUILD_WITH_BUILDAH` env var is still supported and can be
used to override the default either way (i.e., set to `0`, it will force
the legacy path).

Author: jlebon

src/cmdlib.sh

@@ -149,13 +149,24 @@ yaml2json() {
 }
 
 should_build_with_buildah() {
+    local variant manifest
     if [ -n "${COSA_BUILD_WITH_BUILDAH:-}" ]; then
         if [ "${COSA_BUILD_WITH_BUILDAH:-}" = 1 ]; then
             return 0
         else
             return 1
         fi
     fi
+    # this slightly duplicates some logic in `prepare_build`, but meh...
+    if [[ -f "src/config.json" ]]; then
+        variant="$(jq --raw-output '."coreos-assembler.config-variant"' src/config.json)"
+        manifest="src/config/manifest-${variant}.yaml"
+    else
+        manifest="src/config/manifest.yaml"
+    fi
+    if [ "$(yq .metadata.build_with_buildah "${manifest}")" = true ]; then
+        return 0
+    fi
     return 1
 }