Add GitHub workflow for building Docker image (#280)

Author: joschi

.github/workflows/docker.yml

@@ -0,0 +1,29 @@
+name: Docker image
+on:
+  push:
+    tags:
+      - '*'
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout SCM
+        uses: actions/checkout@v2.4.0
+      - 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: ${{ secrets.DOCKER_HUB_PASSWORD }}
+      - name: Set Git tag in output
+        id: vars
+        run: echo ::set-output name=tag::${GITHUB_REF#refs/*/}
+      - name: Build & Push Docker image
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          push: true
+          tags: |
+            openapitools/openapi-diff:${{ steps.vars.outputs.tag }}
+            openapitools/openapi-diff:latest