👷 ci(docker): add GitHub Actions workflow for Docker image

- create workflow to build and push Docker images on push to main or tags
- configure Docker Buildx and login to GitHub Container Registry
- use secrets for secure authentication
- tag images with latest and commit SHA for versioning

.github/workflows/docker.yml

@@ -0,0 +1,39 @@
+name: Build and Push Docker Image
+
+on:
+  push:
+    branches: [ main ]
+    tags: [ 'v*' ]
+
+permissions:
+  contents: read
+  packages: write
+
+env:
+  IMAGE_NAME: ghcr.io/${{ github.repository_owner }}/server-verwaltung
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GHCR
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: true
+          tags: |
+            ${{ env.IMAGE_NAME }}:latest
+            ${{ env.IMAGE_NAME }}:${{ github.sha }}