ci: Ensure release-please triggers docker/build-push-action jobs (#443)

* This wasn't automatic up to now because these jobs were in a separate
  workflow and the tagging or release events that occur in release.yml
  cannot trigger children workflows from a mere `secrets.GITHUB_TOKEN`

* href: https://docs.github.com/en/actions/learn-github-actions/events-that-trigger-workflows#triggering-new-workflows-using-a-personal-access-token

.github/workflows/release.yml

@@ -120,3 +120,68 @@ jobs:
       - name: Publish to opam
         run: |
           ./scripts/opam-publish.exp "${{ needs.release-please.outputs.tag_name }}" "$opam_repo" "$source_repo"
+
+  # Note: you may want to update the jobs below and deploy-master.yml at once
+  push_server:
+    needs: [release-please]
+    if: ${{ needs.release-please.outputs.release_created }}
+    name: Push learn-ocaml image to Docker Hub
+    runs-on: ubuntu-latest
+    steps:
+      - name: Get tag name
+        run: tag="${{ needs.release-please.outputs.tag_name }}"; echo "::set-output name=tag::${tag#v}"
+        id: tag
+      - name: Push to Docker Hub
+        uses: docker/build-push-action@v1
+        with:
+          always_pull: true
+          add_git_labels: true
+          labels: "org.opencontainers.image.version=${{ steps.tag.outputs.tag }}"
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+          repository: ocamlsf/learn-ocaml
+          tags: "latest,${{ steps.tag.outputs.tag }}"
+  push_client:
+    needs: [release-please]
+    if: ${{ needs.release-please.outputs.release_created }}
+    name: Push learn-ocaml-client image to Docker Hub
+    runs-on: ubuntu-latest
+    steps:
+      - name: Get tag name
+        run: tag="${{ needs.release-please.outputs.tag_name }}"; echo "::set-output name=tag::${tag#v}"
+        id: tag
+      - name: Push to Docker Hub
+        uses: docker/build-push-action@v1
+        with:
+          always_pull: true
+          add_git_labels: true
+          labels: "org.opencontainers.image.version=${{ steps.tag.outputs.tag }}"
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+          repository: ocamlsf/learn-ocaml-client
+          target: client
+          tags: "latest,${{ steps.tag.outputs.tag }}"
+  push_emacs_client:
+    needs:
+      - release-please
+      - push_client
+    if: ${{ needs.release-please.outputs.release_created }}
+    name: Push emacs-learn-ocaml-client image to Docker Hub
+    runs-on: ubuntu-latest
+    steps:
+      - name: Get tag name
+        run: tag="${{ needs.release-please.outputs.tag_name }}"; echo "::set-output name=tag::${tag#v}"
+        id: tag
+      - name: Push to Docker Hub
+        # https://github.com/docker/build-push-action/tree/releases/v1#readme
+        uses: docker/build-push-action@v1
+        with:
+          path: ci/docker-emacs-learn-ocaml-client
+          build_args: "base=ocamlsf/learn-ocaml-client,version=${{ steps.tag.outputs.tag }}"
+          always_pull: true
+          add_git_labels: true
+          labels: "org.opencontainers.image.version=${{ steps.tag.outputs.tag }}"
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+          repository: ocamlsf/emacs-learn-ocaml-client
+          tags: ${{ steps.tag.outputs.tag }}