Move to official horizon docker image

[encryptio]

rethinkdb/horizon is now a thing, but as of this writing, it does not include our patches in the hzc branch to the client, and it makes some assumptions that are not true in our use case (notably, that there is an app directory to serve.)

• [ ] Open a PR and get the client changes reviewed merged in
• [ ] Figure out how to make the lack of an app directory work
• [ ] Update the deployment to use the official image and pass the arguments in the form it prefers
• [ ] Update the k exec-like code to run hz in its new location

[dalanmiller]

Figure out how to make the lack of an app directory work

I think in your case you're going to override the CMD anyway since the hz serve flags required for HZC are very specific. This shouldn't be a problem I foresee.