Add GitHub workflow for automated Docker image builds

Creates workflow that triggers on version tags (v*) to automatically build
and publish Docker images to GitHub Container Registry. Non-alpha releases
also create draft GitHub releases for review before publishing.

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

.github/workflows/docker-publish.yml

@@ -0,0 +1,75 @@
+name: Build and Publish Docker Image
+
+on:
+  push:
+    tags:
+      - 'v*'
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+      packages: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata (tags, labels)
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
+            type=raw,value=latest,enable=${{ !contains(github.ref_name, 'alpha') }}
+          labels: |
+            org.opencontainers.image.title=Windmill Git Sync
+            org.opencontainers.image.description=Containerized service for synchronizing Windmill workspaces to Git repositories
+            org.opencontainers.image.vendor=${{ github.repository_owner }}
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+
+      - name: Create draft release
+        if: ${{ !contains(github.ref_name, 'alpha') }}
+        uses: softprops/action-gh-release@v1
+        with:
+          draft: true
+          generate_release_notes: true
+          body: |
+            ## Docker Image
+
+            Pull this release:
+            ```bash
+            docker pull ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ github.ref_name }}
+            ```
+
+            Or use the latest tag:
+            ```bash
+            docker pull ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:latest
+            ```