Update workflow: push tagged docker builds (#161)

* test new build workflow

* fix build workflow

* update docs

.github/workflows/build.yml

@@ -0,0 +1,42 @@
+name: Build and push Docker image
+
+on:
+  push:
+    branches:
+      - '*'
+    tags:
+      - 'v*'
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build:
+    runs-on: ubuntu-20.04
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v1
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata (tags, labels) for Docker
+        id: meta
+        uses: docker/metadata-action@v3
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+
+      - name: Build and push image
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          file: docker/Dockerfile.prod
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          push: ${{ startsWith(github.ref, 'refs/tags/v') || endsWith(github.ref, '/stage') }}