ci: add backend image build to docker workflow

Adds a second build+push step for the backend image (valknar/sexy-backend)
using Dockerfile.backend. Both images share the same tag strategy and
separate build caches. Summary step updated to show both images.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -20,6 +20,7 @@ on:
 env:
   REGISTRY: dev.pivoine.art
   IMAGE_NAME: valknar/sexy
+  BACKEND_IMAGE_NAME: valknar/sexy-backend
 
 jobs:
   build-and-push:
@@ -67,10 +68,11 @@ jobs:
             org.opencontainers.image.vendor=valknar
             org.opencontainers.image.source=https://dev.pivoine.art/${{ gitea.repository }}
 
-      - name: Build and push Docker image
+      - name: Build and push frontend Docker image
         uses: docker/build-push-action@v5
         with:
           context: .
+          dockerfile: Dockerfile
           platforms: linux/amd64
           push: ${{ gitea.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
@@ -81,32 +83,77 @@ jobs:
             NODE_ENV=production
             CI=true
 
+      - name: Extract metadata for backend image
+        id: meta-backend
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.BACKEND_IMAGE_NAME }}
+          tags: |
+            type=raw,value=latest,enable={{is_default_branch}}
+            type=ref,event=branch
+            type=ref,event=pr
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
+            type=sha,prefix={{branch}}-
+            type=raw,value=${{ gitea.event.inputs.tag }},enable=${{ gitea.event_name == 'workflow_dispatch' }}
+          labels: |
+            org.opencontainers.image.title=sexy.pivoine.art backend
+            org.opencontainers.image.description=GraphQL backend for sexy.pivoine.art (Fastify + Drizzle + Pothos)
+            org.opencontainers.image.vendor=valknar
+            org.opencontainers.image.source=https://dev.pivoine.art/${{ gitea.repository }}
+
+      - name: Build and push backend Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          dockerfile: Dockerfile.backend
+          platforms: linux/amd64
+          push: ${{ gitea.event_name != 'pull_request' }}
+          tags: ${{ steps.meta-backend.outputs.tags }}
+          labels: ${{ steps.meta-backend.outputs.labels }}
+          cache-from: type=registry,ref=${{ env.REGISTRY }}/${{ env.BACKEND_IMAGE_NAME }}:buildcache
+          cache-to: type=registry,ref=${{ env.REGISTRY }}/${{ env.BACKEND_IMAGE_NAME }}:buildcache,mode=max
+          build-args: |
+            NODE_ENV=production
+            CI=true
+
       - name: Generate image digest
         if: gitea.event_name != 'pull_request'
         run: |
-          echo "### Docker Image Published :rocket:" >> $GITEA_STEP_SUMMARY
+          echo "### Docker Images Published :rocket:" >> $GITEA_STEP_SUMMARY
           echo "" >> $GITEA_STEP_SUMMARY
           echo "**Registry:** \`${{ env.REGISTRY }}\`" >> $GITEA_STEP_SUMMARY
-          echo "**Image:** \`${{ env.IMAGE_NAME }}\`" >> $GITEA_STEP_SUMMARY
           echo "" >> $GITEA_STEP_SUMMARY
-          echo "**Tags:**" >> $GITEA_STEP_SUMMARY
+          echo "**Frontend (\`${{ env.IMAGE_NAME }}\`):**" >> $GITEA_STEP_SUMMARY
           echo "\`\`\`" >> $GITEA_STEP_SUMMARY
           echo "${{ steps.meta.outputs.tags }}" >> $GITEA_STEP_SUMMARY
           echo "\`\`\`" >> $GITEA_STEP_SUMMARY
           echo "" >> $GITEA_STEP_SUMMARY
-          echo "**Pull command:**" >> $GITEA_STEP_SUMMARY
+          echo "**Backend (\`${{ env.BACKEND_IMAGE_NAME }}\`):**" >> $GITEA_STEP_SUMMARY
+          echo "\`\`\`" >> $GITEA_STEP_SUMMARY
+          echo "${{ steps.meta-backend.outputs.tags }}" >> $GITEA_STEP_SUMMARY
+          echo "\`\`\`" >> $GITEA_STEP_SUMMARY
+          echo "" >> $GITEA_STEP_SUMMARY
+          echo "**Pull commands:**" >> $GITEA_STEP_SUMMARY
           echo "\`\`\`bash" >> $GITEA_STEP_SUMMARY
           echo "docker pull ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:latest" >> $GITEA_STEP_SUMMARY
+          echo "docker pull ${{ env.REGISTRY }}/${{ env.BACKEND_IMAGE_NAME }}:latest" >> $GITEA_STEP_SUMMARY
           echo "\`\`\`" >> $GITEA_STEP_SUMMARY
 
-      - name: PR Comment - Image built but not pushed
+      - name: PR Comment - Images built but not pushed
         if: gitea.event_name == 'pull_request'
         run: |
-          echo "### Docker Image Built Successfully :white_check_mark:" >> $GITEA_STEP_SUMMARY
+          echo "### Docker Images Built Successfully :white_check_mark:" >> $GITEA_STEP_SUMMARY
           echo "" >> $GITEA_STEP_SUMMARY
-          echo "Image was built successfully but **not pushed** (PR builds are not published)." >> $GITEA_STEP_SUMMARY
+          echo "Images were built successfully but **not pushed** (PR builds are not published)." >> $GITEA_STEP_SUMMARY
           echo "" >> $GITEA_STEP_SUMMARY
-          echo "**Would be tagged as:**" >> $GITEA_STEP_SUMMARY
+          echo "**Frontend would be tagged as:**" >> $GITEA_STEP_SUMMARY
           echo "\`\`\`" >> $GITEA_STEP_SUMMARY
           echo "${{ steps.meta.outputs.tags }}" >> $GITEA_STEP_SUMMARY
           echo "\`\`\`" >> $GITEA_STEP_SUMMARY
+          echo "" >> $GITEA_STEP_SUMMARY
+          echo "**Backend would be tagged as:**" >> $GITEA_STEP_SUMMARY
+          echo "\`\`\`" >> $GITEA_STEP_SUMMARY
+          echo "${{ steps.meta-backend.outputs.tags }}" >> $GITEA_STEP_SUMMARY
+          echo "\`\`\`" >> $GITEA_STEP_SUMMARY