Consolidate run commands into a single layer

[danrubins]

By putting the cleanup script in the same layer as the install script, we can avoid persisting temporary filesystem changes in the intermediate layers. Between this and #43, it trims the image size by about a third.

[abernix]

This is great! Given my comments on #43, do you know which of these makes the more substantial improvement?

I'm pretty sure we could make tremendous improvements by switching to multi-stage Docker builds, if you were willing to take a look at that.

Given its popularity, I think improving this image is very much welcomed and I thank you for looking into it. Though I will be honest, I have a lot of respect for the image produced by https://github.com/disney/meteor-base, which offers a lot in terms of improvement since it doesn't try to be a compatibility fix for mup users like this image does. That luxury provides a lot in terms of improving performance by not needing to be considerate of backwards compatibility!