Create docker-release.yml

Push images to Docker Hub for a tagged version.

.github/workflows/docker-release.yml

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+# This is a basic workflow to help you get started with Actions
+
+name: Publish tagged images to Docker
+
+# Controls when the workflow will run
+on:
+  # Triggers the workflow on push or pull request events but only for the develop branch
+  push:
+    tags: 
+      - '*'
+#   pull_request:
+#     branches: [ develop ]
+
+  # Allows you to run this workflow manually from the Actions tab
+  workflow_dispatch:
+
+# A workflow run is made up of one or more jobs that can run sequentially or in parallel
+jobs:
+  # This workflow contains a single job called "build"
+  build:
+    # The type of runner that the job will run on
+    runs-on: ubuntu-latest
+
+    # Steps represent a sequence of tasks that will be executed as part of the job
+    steps:
+      # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
+      - name: Checkout Repo
+        uses: actions/checkout@v2
+
+      - name: Set tag
+        id: vars
+        run: echo ::set-output name=tag::${GITHUB_REF#refs/*/}
+
+      - name: Login to Docker Hub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKER_HUB_USERNAME }}
+          password: ${{ secrets.DOCKER_HUB_ACCESS_TOKEN }}
+
+      - name: Set up Docker Buildx
+        id: buildx
+        uses: docker/setup-buildx-action@v1
+
+      - name: Build and push
+        id: docker_build
+        uses: docker/build-push-action@v2
+        with:
+          context: ./
+          file: ./Dockerfile
+          push: true
+          tags: ecmwf/magics:${{ steps.vars.outputs.tag }}
+
+      - name: Image digest
+        run: echo ${{ steps.docker_build.outputs.digest }}