Allow changing the base Docker image from ubuntu:16.04

[C0deH4cker]

Especially with glibc heap exploitation challenges, it's important for CTF challenge authors to have control over which base image is used. This will also help address the issue where a challenge is linked against a newer version of glibc than is present in the container so it fails to run.

[C0deH4cker]

This is actively being worked on and should hopefully be merged soon!