[misc] Build release branches in the Release workflow

Pushes to release-* branches now run the Release workflow and publish
immutable sha-* container images, the way pushes to main already do.
Without this, a release branch had no post-merge CI and no artifact to
run the pre-release test suites against before tagging.

The floating "main" image tag is only pushed for main builds; release
branches get the sha-* tag alone.

.github/workflows/release.yml

@@ -6,6 +6,7 @@ on:
       - "v*"
     branches:
       - main
+      - "release-*"
   pull_request:
 
 env:
@@ -254,15 +255,19 @@ jobs:
         id: tag_and_push_images
         if: |
           (github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name == github.repository) ||
-          (github.event_name == 'push' && github.ref == 'refs/heads/main')
+          (github.event_name == 'push' && (github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/heads/release-')))
         run: |
           set -euo pipefail
 
           resolve_tags() {
             if [[ "${{ github.event_name }}" == "pull_request" ]]; then
               echo "pr-${{ github.event.pull_request.number }}"
-            else
+            elif [[ "${{ github.ref }}" == "refs/heads/main" ]]; then
               echo "main sha-$(git rev-parse --short HEAD)"
+            else
+              # Release branches get an immutable sha-* tag only — the floating
+              # "main" tag must never move from a release branch.
+              echo "sha-$(git rev-parse --short HEAD)"
             fi
           }