github: add workflow to build images for ghcr.io

.github/workflows/alpine-release.yml

@@ -0,0 +1,33 @@
+name: release docker image
+on:
+  push:
+    tags:
+env:
+  IMAGE_NAME: "gmid"
+jobs:
+  build:
+    permissions: write-all
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+
+    - name: build the image
+      run: docker build -f contrib/Dockerfile -t gmid:alpine .
+
+    - name: login to ghcr.io
+      uses: docker/login-action@v2
+      with:
+        registry: ghcr.io
+        username: ${{ github.actor }}
+        password: ${{ secrets.GITHUB_TOKEN }}
+
+    - name: push the image
+      run: |
+        IMAGE_ID=ghcr.io/${{ github.repository_owner }}/$IMAGE_NAME
+        IMAGE_ID=$(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 gmid:alpine $IMAGE_ID:$VERSION
+        docker push $IMAGE_ID:$VERSION