Use regctl to read MW version label without pulling image (#596)

Replace docker pull + docker inspect with regctl image config to
extract the MW_VERSION label from CanastaBase. This reads the label
directly from the registry manifest (~KB) instead of downloading the
full ~700MB image, reducing the step from ~30s to a few seconds.

Author: cicalese

.github/workflows/docker-image.yml

@@ -40,14 +40,16 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v3
+      -
+        name: Install regctl
+        uses: regclient/actions/regctl-installer@v0.1
       -
         name: Extract MW_VERSION from CanastaBase labels
         id: extract_mw
         run: |
           set -euo pipefail
           BASE_IMAGE=$(awk -F'=' '/^ARG BASE_IMAGE=/ {print $2}' Dockerfile)
-          docker pull "$BASE_IMAGE" >/dev/null
-          MW_VERSION=$(docker inspect --format '{{ index .Config.Labels "wiki.canasta.mediawiki.version" }}' "$BASE_IMAGE")
+          MW_VERSION=$(regctl image config "$BASE_IMAGE" | jq -r '.config.Labels["wiki.canasta.mediawiki.version"]')
           echo "MW_VERSION=$MW_VERSION"
           echo "MW_VERSION=$MW_VERSION" >> $GITHUB_ENV