Perform best practice of cleaning up apt-get files
Perform best practice of cleaning up apt-get files

From https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/

Image shrunk from 365.2 -> 327.7 MB

gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab