Update docker base image to correct version

[davidcallanan]

This is the potential solution to #2 x86_64-elf-gcc: not found

The docker base image is the latest version of an image provided by another user. This image has likely since been updated, causing problems.

Todo: I will fork the correct version of this base image. I will then update the Dockerfile to reference this new base image.

[davidcallanan]

The base image is working fine for me after re-cloning and fresh install of docker. So this problem seems to only be happening for some -- not sure how that's possible given the exact same docker image is being used. Need to keep looking at this.

I will probably still fork the image anyway to prevent any future potential issues.

Edit: The gcc error only applies to episode 2, I accidentally tested episode 1. I will test for episode 2 shortly.

Edit 2: Tested for episode 2, there is still no error. Not sure how others are getting this error.