Ensure that package managers clean up

Every package manager should clean up after themselves in order to keep
docker layers neat and tiny:

apt (Debian/Ubuntu package manager):

 - unneeded dependencies are cleared (autoremove)
 - caches are cleaned (clean)
 - package lists are deleted

yum (CentOS package manager),
zypper (OpenSuse package manager)

 - should clear caches after installing dependencies

pip (Python package manager),
apk (Alpine package manager)

 - should use no cache for installing dependencies

Dockerfile.omnibus-gitlab-xenial

@@ -2,6 +2,8 @@ FROM ubuntu:16.04
 
 RUN apt-get update -q \
         && apt-get install -y g++ gcc git make curl zlib1g-dev libssl-dev locales \
+        && apt-get autoremove -yq \
+        && apt-get clean -yqq \
         && rm -rf /var/lib/apt/lists/*
 
 RUN echo "en_US.UTF-8 UTF-8" > /etc/locale.gen