ci: publish Docker images for version tags and stable releases

- Publish images on semantic version tags (vX.Y.Z) as :vX.Y.Z and :latest
- Publish :stable image via workflow_run after successful Stable Tag workflow
- Build stable image from exact commit marked as stable
- Remove duplicate build steps and unify tag computation

https://chatgpt.com/share/694b9758-58fc-800f-a586-8f3a341ece9d

.github/workflows/publish-image.yml

@@ -3,16 +3,19 @@ name: publish-image
 on:
   push:
     tags:
-      - "*"
+      - "v*.*.*"
+
+  workflow_run:
+    workflows: ["Stable Tag"]   # MUST match stable-tag.yml -> name: Stable Tag
+    types: [completed]
 
 jobs:
-  tests:
-    uses: ./.github/workflows/reusable-test.yml
-
   build-and-push:
-    needs: tests
-    runs-on: ubuntu-latest
+    if: |
+      (github.event_name == 'push') ||
+      (github.event_name == 'workflow_run' && github.event.workflow_run.conclusion == 'success')
 
+    runs-on: ubuntu-latest
     permissions:
       contents: read
       packages: write
@@ -20,6 +23,10 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+        with:
+          # push: checks out the tag ref
+          # workflow_run: checks out the exact commit that the Stable Tag workflow ran on
+          ref: ${{ github.event_name == 'workflow_run' && github.event.workflow_run.head_sha || github.ref }}
 
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v3
@@ -27,20 +34,6 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
-      - name: Extract tag
-        id: meta
-        run: |
-          echo "tag=${GITHUB_REF_NAME}" >> "$GITHUB_OUTPUT"
-
-      - name: Check semver tag
-        id: semver
-        run: |
-          if [[ "${GITHUB_REF_NAME}" =~ ^v?[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
-            echo "is_semver=true" >> "$GITHUB_OUTPUT"
-          else
-            echo "is_semver=false" >> "$GITHUB_OUTPUT"
-          fi
-
       - name: Login to GHCR
         uses: docker/login-action@v3
         with:
@@ -48,27 +41,27 @@ jobs:
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
-      - name: Build and push (tag)
-        uses: docker/build-push-action@v6
-        with:
-          context: .
-          file: ./Dockerfile
-          push: true
-          platforms: linux/amd64,linux/arm64
-          tags: |
-            ghcr.io/${{ github.repository }}:${{ steps.meta.outputs.tag }}
-          cache-from: type=gha
-          cache-to: type=gha,mode=max
+      - name: Compute tags
+        id: meta
+        shell: bash
+        run: |
+          set -euo pipefail
+          IMAGE="ghcr.io/${{ github.repository }}"
 
-      - name: Build and push (latest)
-        if: steps.semver.outputs.is_semver == 'true'
+          if [ "${{ github.event_name }}" = "push" ]; then
+            TAG="${{ github.ref_name }}"     # e.g. v1.1.0
+            echo "tags=$IMAGE:$TAG,$IMAGE:latest" >> "$GITHUB_OUTPUT"
+          else
+            echo "tags=$IMAGE:stable" >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Build and push
         uses: docker/build-push-action@v6
         with:
           context: .
           file: ./Dockerfile
           push: true
           platforms: linux/amd64,linux/arm64
-          tags: |
-            ghcr.io/${{ github.repository }}:latest
+          tags: ${{ steps.meta.outputs.tags }}
           cache-from: type=gha
           cache-to: type=gha,mode=max