chore: streamline Docker workflow with attestation support

Updated GitHub Actions workflow based on kit.pivoine.art template:

Permissions:
- Added id-token: write (required for attestations)
- Added attestations: write (enables build provenance)

Improvements:
- Added workflow_dispatch trigger for manual runs
- Updated docker/build-push-action from v5 to v6
- Added conditional login (skip on pull requests)
- Added artifact attestation step with actions/attest-build-provenance@v2
- Generates and pushes build provenance to registry
- Provides supply chain security and transparency

Attestation benefits:
- Verifiable build provenance
- SLSA (Supply chain Levels for Software Artifacts) compliance
- Cryptographically signed metadata about build process
- Helps users verify image authenticity

The workflow now matches modern Docker image publishing best practices
with full attestation support for enhanced security.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/docker-build-push.yml

@@ -9,6 +9,7 @@ on:
   pull_request:
     branches:
       - main
+  workflow_dispatch:
 
 env:
   REGISTRY: ghcr.io
@@ -20,6 +21,8 @@ jobs:
     permissions:
       contents: read
       packages: write
+      id-token: write
+      attestations: write
 
     steps:
       - name: Checkout repository
@@ -28,7 +31,8 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
-      - name: Log in to GitHub Container Registry
+      - name: Log into registry ${{ env.REGISTRY }}
+        if: github.event_name != 'pull_request'
         uses: docker/login-action@v3
         with:
           registry: ${{ env.REGISTRY }}
@@ -50,7 +54,8 @@ jobs:
             type=raw,value=latest,enable={{is_default_branch}}
 
       - name: Build and push Docker image
-        uses: docker/build-push-action@v5
+        id: build-and-push
+        uses: docker/build-push-action@v6
         with:
           context: .
           push: ${{ github.event_name != 'pull_request' }}
@@ -60,5 +65,10 @@ jobs:
           cache-to: type=gha,mode=max
           platforms: linux/amd64,linux/arm64
 
-      - name: Image digest
+      - name: Generate artifact attestation
-        run: echo ${{ steps.meta.outputs.digest }}
+        if: github.event_name != 'pull_request'
+        uses: actions/attest-build-provenance@v2
+        with:
+          subject-name: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          subject-digest: ${{ steps.build-and-push.outputs.digest }}
+          push-to-registry: true