Add docker build workflow

.github/workflows/docker.yml

@@ -0,0 +1,39 @@
+name: Docker
+
+on:
+  push:
+    # Publish `adap-cw*` tags as releases.
+    tags:
+        - adap-cw*
+
+env:
+  IMAGE_NAME: wahlzeit
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Build image
+        run: docker build . --file Dockerfile --tag $IMAGE_NAME
+
+      - name: Log into GitHub Container Registry
+        run: echo "${{ secrets.GITHUB_TOKEN }}" | docker login docker.pkg.github.com -u ${{ github.actor }} --password-stdin
+
+      - name: Push image to GitHub Container Registry
+        run: |
+          IMAGE_ID=docker.pkg.github.com/$GITHUB_REPOSITORY/$IMAGE_NAME
+
+          # Change all uppercase to lowercase
+          IMAGE_ID=$(echo $IMAGE_ID | tr '[A-Z]' '[a-z]')
+
+          # Strip git ref prefix from version
+          VERSION=$(echo "${{ github.ref }}" | sed -e 's,.*/\(.*\),\1,')
+
+          echo IMAGE_ID=$IMAGE_ID
+          echo VERSION=$VERSION
+
+          docker tag $IMAGE_NAME $IMAGE_ID:$VERSION
+          docker tag $IMAGE_NAME $IMAGE_ID:latest
+          docker push $IMAGE_ID:$VERSION