Useless 'apt-get update' in challenge/Dockerfile

## Issue:
In [challenge/Dockerfile](https://github.com/LiveOverflow/pwn_docker_example/blob/master/challenge/Dockerfile), the command `RUN apt-get update` is useless since the command updates the packages list, but the packages are not upgraded later.

## Solution:
Add `apt-get upgrade` to the RUN layer, or remove the layer.