Possible improvements

At @marc-barry's demand, here a couple things that could make usage simpler.

### dedicated command for initial setup & migrations

In the readme you give a command to setup the db: https://github.com/digitalocean/docker-shipit-engine#database-management

Would be great if this could be done in a single command handled by the entry point, e.g. `docker run shipit setup`, and possibly `docker run shipit upgrade` to run post upgrade.

### pubsubstub might cause issues in this setup

Something that's a bit finicky with Shipit is the SSE connection handling. Long story short client will keep a long lived connection per stack page. These aren't very expensive, but if your Puma config is a bit too strict in term of threads, you can easily hold all thread.

The ideal setup is to have dedicated containers for handling the streaming endpoints, see: https://github.com/Shopify/shipit-engine/issues/496#issuecomment-351429330 for more details. But that makes for a more complicated setup as you need a LB etc.

Let me know if you need more details on that.

### Database

Maybe I misread, but it seems you are focusing on postgres, I guess it's fine, but I suppose MySQL could be supported as well (and is a much better tested solution with Shipit).

I'll update this issue if something else comes up.