Release latest Docker image every dev release (#3747)

.github/workflows/build.yml

@@ -285,6 +285,19 @@ jobs:
         uses: pypa/gh-action-pypi-publish@master
         with:
           password: ${{ secrets.PYPI_API_TOKEN }}
+      - name: Build Docker release
+          run: |
+            docker build -t motoserver/moto . --tag moto:latest
+      - name: Login to DockerHub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+      - name: Build and push
+        uses: docker/build-push-action@v2
+        with:
+          push: true
+          tags: motoserver/moto:latest
       - name: Get version number
         run: |
           version=$(grep -oP '(?<=__version__ = ")[0-9.a-z]+(?=")' moto/__init__.py)