Integrate building Docker image in release workflow

Refs #280

Author: joschi

.github/workflows/docker.yml

@@ -23,6 +23,7 @@ jobs:
         with:
           distribution: 'zulu'
           java-version: 11
+          cache: 'maven'
           # Value of the distributionManagement/repository/id field of the pom.xml
           server-id: sonatype-nexus-staging
           gpg-private-key: ${{ secrets.GPG_PRIVATE_KEY }}
@@ -48,3 +49,24 @@ jobs:
         run: |
           ./mvnw -B release:rollback -Prelease -Dgpg.passphrase=${{secrets.GPG_PASSPHRASE}}
           echo "You may need to manually delete the GitHub tag, if it was created."
+  docker:
+    needs: release
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2.4.0
+        ref: ${{ github.event.inputs.releaseVersion }}
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+      - name: Login to DockerHub
+        uses: docker/login-action@v1 
+        with:
+          username: ${{ secrets.DOCKER_HUB_USERNAME }}
+          password: ${{ secrets.DOCKER_HUB_PASSWORD }}
+      - name: Build & Push Docker image
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          push: true
+          tags: |
+            openapitools/openapi-diff:${{ github.event.inputs.releaseVersion }}
+            openapitools/openapi-diff:latest