Enhance release workflow and add local testing script for registry images

- Update release.yaml to publish :latest and :prerelease-tag for stable and pre-release tags respectively.
- Add README section for local testing of registry images with a new script.
- Introduce scripts/test-registry-image.sh for end-to-end testing of published images.

.gitea/workflows/release.yaml

@@ -48,18 +48,20 @@ jobs:
 
           # Note: This repository only builds and pushes images.
           # FluxCD Image Automation in another repository performs deployment.
-          # Stable tags (vX.Y.Z) may become :latest. Pre-release tags never do.
-          if [[ "$TAG" =~ ^v[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
+          # Stable tags (vX.Y.Z) also publish :latest.
+          # Pre-release tags (e.g. vX.Y.Z-rc1) also publish :prerelease-tag.
+          if printf '%s' "$TAG" | grep -Eq '^v[0-9]+\.[0-9]+\.[0-9]+$'; then
             docker buildx build \
               --platform linux/amd64 \
               --push \
-              --tag $REGISTRY/$IMAGE_NAME:$TAG \
-              --tag $REGISTRY/$IMAGE_NAME:latest \
+              --tag "$REGISTRY/$IMAGE_NAME:$TAG" \
+              --tag "$REGISTRY/$IMAGE_NAME:latest" \
               .
           else
             docker buildx build \
               --platform linux/amd64 \
               --push \
-              --tag $REGISTRY/$IMAGE_NAME:$TAG \
+              --tag "$REGISTRY/$IMAGE_NAME:$TAG" \
+              --tag "$REGISTRY/$IMAGE_NAME:prerelease-tag" \
               .
           fi