Various housekeeping to speed up iteration

[jessberg]

It's rather cumbersome to iterate on the collector and more broadly the whole system right now. Here are a few things to look into to speed up iteration.

1. It takes a while to rebuild the collector from scratch. It's not a ton of time, about 5 minutes, but it does slow down iteration. How can we mitigate this? I have commented out various modules we do not use in docker_config.yaml, and although that does speed things up, it also means if you don't know the build process you may think certain modules are included in the image when they aren't. Another option for this might be caching the go dependencies when building the docker image.
2. Pretty often, you need to run "docker system prune" or there is not enough space. It doesn't take forever to run this, but it does take some time and is cumbersome. Is there a way to auto-prune images we didn't even tag and obviously aren't using?