Add Docker build workflow to publish images to GHCR

Adds a GitHub Actions workflow that builds and pushes Docker images
to ghcr.io/kuberik/github-controller when version tags are pushed.
This ensures the install manifests published by the release workflow
reference images that actually exist in the registry.

Author: LittleChimera

.github/workflows/docker-build.yml

@@ -0,0 +1,43 @@
+name: Docker Build and Push
+
+on:
+  push:
+    tags:
+      - "v*.*.*"
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - name: Checkout
+        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: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract version from tag
+        id: meta
+        run: |
+          VERSION=${GITHUB_REF#refs/tags/}
+          echo "version=${VERSION}" >> $GITHUB_OUTPUT
+          echo "Building version: ${VERSION}"
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: true
+          tags: ghcr.io/kuberik/github-controller:${{ steps.meta.outputs.version }}
+          platforms: linux/amd64,linux/arm64
+          cache-from: type=gha
+          cache-to: type=gha,mode=max