message: build_container: add link to Docker Hub for tags.
  Avoid having to look this up when bumping envoy_build_sha.sh.
diff: Envoy's CI has a build run called `build_image`. On a commit to master, `ci/build_container/docker_push.sh`
  checks if the commit has changed the `ci/build_container` directory. If there are changes, CI builds a new `lyft/envoy-build`
  -image. The image is pushed to dockerhub under `latest` and under the commit sha.
  +image. The image is pushed to [dockerhub](https://hub.docker.com/r/lyft/envoy-build/tags/) under `latest` and under the commit sha.
  After the PR that changes `ci/build_container` has been merged, and the new image gets pushed,
  a second PR is needed to update `ci/envoy_build_sha.sh`. In order to pull the new tagged version of
  