Detect Dockerfile changes to decide when to rebuild all versions

On PRs and pushes, only build versions that don't exist in GHCR
yet. If the Dockerfile itself changed, rebuild all versions to
validate (without pushing on PRs).

Author: baelter

.github/workflows/ruby.yml

@@ -9,6 +9,7 @@ on:
   pull_request:
     paths:
       - ruby/Dockerfile
+      - .github/workflows/ruby.yml
   workflow_dispatch:
     inputs:
       force:
@@ -41,6 +42,20 @@ jobs:
 
       - uses: docker/setup-buildx-action@v4
 
+      - name: Check if Dockerfile changed
+        id: dockerfile
+        run: |
+          if [ "${{ github.event_name }}" = "pull_request" ]; then
+            BASE="${{ github.event.pull_request.base.sha }}"
+          else
+            BASE="${{ github.event.before }}"
+          fi
+          if git diff --name-only "$BASE" "${{ github.sha }}" -- ruby/Dockerfile | grep -q .; then
+            echo "changed=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "changed=false" >> "$GITHUB_OUTPUT"
+          fi
+
       - name: Check if image exists
         id: check
         run: |
@@ -51,14 +66,14 @@ jobs:
           fi
 
       - uses: docker/login-action@v4
-        if: github.event_name != 'pull_request' && (steps.check.outputs.exists != 'true' || (github.event_name == 'workflow_dispatch' && inputs.force))
+        if: github.event_name != 'pull_request' && (steps.check.outputs.exists != 'true' || steps.dockerfile.outputs.changed == 'true' || (github.event_name == 'workflow_dispatch' && inputs.force))
         with:
           registry: ${{ env.REGISTRY }}
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - uses: docker/build-push-action@v7
-        if: github.event_name == 'pull_request' || steps.check.outputs.exists != 'true' || (github.event_name == 'workflow_dispatch' && inputs.force)
+        if: steps.check.outputs.exists != 'true' || steps.dockerfile.outputs.changed == 'true' || (github.event_name == 'workflow_dispatch' && inputs.force)
         with:
           context: ruby
           platforms: linux/amd64,linux/arm64