Get rid of Dockerfile.public

[bhearsum]

After #1304 lands, Dockerfile and Dockerfile.public will be identical except for their CMD. These can be fairly easily combined by setting /app/docker.d/init.sh as the ENTRYPOINT, and then ensuring anything that starts instances sets the appropriate CMD. For docker-compose.yml this is easy, but we also need to deal with our staging and production deployments, which do not set either currently. Before we can do this I suggest that we make the following changes to them:

• Pull the same image for both admin and public
• Configure k8s to set the appropriate CMD for each (it's probably best to delegate ENTRYPOINT to the Dockerfile, although I could be convinced otherwise)

I don't see any way to do this as a non-breaking change, so we'll want to have changes to this repo and cloudops-infra ready to go around the same time. Changes to either one of them are likely to trigger a deploy, so I suggest we land as follows:

• Push the cloudops-infra change first -- which will almost certainly fail to deploy due to the current images lacking an ENTRYPOINT
• Push the change to this repo. That should trigger a deploy that will then succeed.