Introduce bootc-base-imagectl

- Embed the manifests into the container image
- Add bootc-base-imagectl which is a tightly controlled frontend
  to execute on those manifests.

For now, we don't attempt to rework how we build the standard
image to actually look like `dnf install`, but we show that
it can work.

Signed-off-by: Colin Walters <walters@verbum.org>

.gitlab-ci.yml

@@ -26,7 +26,7 @@ build-image:
     matrix:
       - TIER: [minimal, standard, tier-x]
   variables:
-    EXTRA_ARGS: "--security-opt=label=disable --cap-add=all --build-arg MANIFEST=fedora-$TIER.yaml"
+    EXTRA_ARGS: "--security-opt=label=disable --cap-add=all --build-arg MANIFEST=fedora-$TIER"
   rules:
     - if: $CI_PROJECT_NAMESPACE != "fedora/bootc"
       when: never