ci: add Docker build + push workflow

Forgejo Actions workflow that builds and pushes Docker images on:
- Push to main → tagged 'latest' + short SHA
- Tags matching 'v*' → semver tags (1.0.0, 1.0, latest)
- PRs → build only (no push)

Pushes to both GHCR and Docker Hub with GitHub Actions cache.

Required secrets:
- DOCKERHUB_USERNAME: Docker Hub username
- DOCKERHUB_PASSWORD: Docker Hub access token
- GITHUB_TOKEN: automatically provided by Forgejo (or set manually for GHCR)

Note: Requires a Forgejo Actions runner to be registered. See:
https://forgejo.org/docs/latest/user/actions/#runner-setup

.forgejo/workflows/docker.yml

@@ -0,0 +1,60 @@
+name: Build and Push Docker Image
+
+on:
+  push:
+    branches: [main]
+    tags: ['v*']
+  pull_request:
+    branches: [main]
+
+env:
+  IMAGE_NAME: kafka
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Docker meta
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: |
+            ghcr.io/${{ github.repository_owner }}/${{ env.IMAGE_NAME }}
+            docker.io/${{ secrets.DOCKERHUB_USERNAME }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=raw,value=latest,enable=${{ github.ref == 'refs/heads/main' }}
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=sha,prefix=
+
+      - name: Login to GHCR
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Login to Docker Hub
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: docker.io
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_PASSWORD }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max