docker logs shows basic documentation

With those changes, when the apt-cacher container runs it prints some basic usage
documentation to the Docker logs.

You can access this doc with:
    docker logs apt-cacher

see gh-1
use DISTRIBUTIONS env to specify distributions to cache

Usage:

    docker run -d -p 3142:3142 -e DISTRIBUTIONS="quantal, trusty" -t apt-cacher

see gh-2
Merge remote-tracking branch 'origin/self-documenting-container'
Merge remote-tracking branch 'origin/distrib-names'

Conflicts:
	Dockerfile
	run.sh
change env variable DISTRIBUTIONS to UBUNTU_RELEASE_NAMES