Don't use composer in container

[brettp]

Currently composer is installed in the Dockerfile and called from the container's run.sh script. There are a few problems with this:

1. It assumes you want the container to manage vendors.
2. It goes against the philosophy of separating tools to manage code from code itself.
3. It causes a sometimes significant delay during startup.

I'm bringing this up because the point of Docker is to run an app with the user not having to worry about external deps. If we drop composer, we'd have to expect the user to have a working Elgg installation means the user either needs to download a release of Elgg that's already composed, or have composer and its reqs installed.