Merge pull request #31 from andrew-waters/example-gcr

Adds GCR example

README.md

@@ -192,4 +192,16 @@ with:
   repository: myorg/myrepository
   tag_with_ref: true
   target: mytarget
-```
+```
+
+The following will build the root Dockerfile, tag the image as `myorg/myrepository:latest`, log in to Google Container Registry using GitHub secrets (where `DOCKER_PASSWORD` is a [JSON key](https://cloud.google.com/container-registry/docs/advanced-authentication#json-key)), and push the image to the CGR repository `myorg/myrepository`:
+
+```yaml
+uses: docker/build-push-action@v1
+with:
+  username: _json_key
+  password: ${{ secrets.DOCKER_PASSWORD }}
+  registry: gcr.io
+  repository: myorg/myrepository
+  tags: latest
+```