Docker cleanup

[michaelarnauts]

Since the last Docker related cleanup, hass gained some weight again. This was mainly due to external packages that had to be compiled from source that didn't cleanup their sources after installing the binaries.

Docker-related changes:

• Completes .dockerignore with files that aren't related for Docker. This is mainly to speed up development in Docker, since the image that dockerhub creates is usually quite clean.
• Sync Dockerfile.dev with Dockerfile (add the uninstall of enum34 to Dockerfile.dev)
• Add a warning in Dockerfile to also update Dockerfile.dev
• Have the Docker dependency build-scripts use /usr/src/app/build/ for temporary build storage, and make sure this folder is cleaned up. All scripts install the binaries to a system path, so the sources can be removed.

Before these changes, the setup_docker_prereqs script added 463 MB to the image. After these changes, it adds 370 MB, so we lost 93 pounds megabytes!

Before:

$ docker history homeassistant/home-assistant | grep virtualization
<missing>           6 days ago          /bin/sh -c virtualization/Docker/setup_doc...   463MB               

After:

$ docker history home-assistant  | grep virtualization
2b934453e141        22 minutes ago      /bin/sh -c virtualization/Docker/setup_doc...   370 MB              

[balloob]

Awesome. Thanks! I'll try to pay better attention when I see PRs come in for Docker.