Cache prior build layers for ~5 days

This vastly improves build performance at the cost of some extra
registry server storage (which there is plenty of).   Note: The cache
TTL needs to be coordinated with the registry server's tag-cleanup
policy.

Signed-off-by: Chris Evich <chris_gitlab@icuc.me>

.gitlab-ci.yml

@@ -47,6 +47,8 @@ build:
         echo "Building/Pushing to: ${CI_REGISTRY_IMAGE}:${IMAGE_TAG}";
     - >-
       buildah build \
+        --layers --cache-from "${CI_REGISTRY_IMAGE}" \
+        --cache-to "${CI_REGISTRY_IMAGE}" --cache-ttl=128h \
         --label "org.opencontainers.image.source=${CI_PROJECT_URL}" \
         --label "org.opencontainers.image.revision=$CI_COMMIT_SHA" \
         --label "org.opencontainers.image.created=$CI_JOB_STARTED_AT" \