Add Dockerfile and Docker Hub publishing (#9)

Adds docker image publishing with example usage in the README.

.github/workflows/docker.yml

@@ -0,0 +1,27 @@
+name: Publish Docker image
+on:
+  push:
+    tags:
+      - v*
+jobs:
+  push_to_registry:
+    name: Push Docker image to Docker Hub
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out the repo
+        uses: actions/checkout@v2
+      - name: Publish version to Registry
+        uses: elgohr/Publish-Docker-Github-Action@master
+        with:
+          name: davegallant/npm-groovy-lint-test
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+          tag_semver: true
+      - name: Publish latest to Registry
+        uses: elgohr/Publish-Docker-Github-Action@master
+        with:
+          name: davegallant/rfd-notify
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+          tags: "latest"
+