build Docker image using GH Actions

since Docker Hub free plan no longer auto-builds against GitHub repos

.github/workflows/build.yml

@@ -0,0 +1,30 @@
+name: Build and Release Image
+
+on:
+  release:
+    types:
+      - created
+
+env:
+  IMAGE_NAME: valhalla
+
+jobs:
+  push:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Build image
+        run: docker build . --tag image
+      - name: Log into registry
+        run: echo "${{ secrets.GITHUB_TOKEN }}" | docker login docker.pkg.github.com -u ${{ github.actor }} --password-stdin
+      - name: Push image
+        run: |
+          IMAGE_ID=docker.pkg.github.com/${{ github.repository }}/$IMAGE_NAME
+          # Strip git ref prefix from version
+          [[ "${{ github.ref }}" == "refs/tags/"* ]] && VERSION=$(echo "${{ github.ref }}" | sed -e 's,.*/\(.*\),\1,')
+          echo IMAGE_ID=$IMAGE_ID
+          echo VERSION=$VERSION
+          docker tag image $IMAGE_ID:$VERSION
+          # Use Docker `latest` tag convention
+          [ "$VERSION" == "master" ] && docker tag image $IMAGE_ID:latest
+          docker push $IMAGE_ID:$VERSION