Add docker[-compose] support

[patcon]

Couldn't get editly working on my local OSX instance, so leaned on docker to get things going.

Added a simple container to persist some state between containers and make it easier to get artefacts out. Final image is quite large (~1GB), but could probably be made smaller. I tried to get it going with alpine linux at first (smaller footprint, likely around 300mb) but had some issues, so just cut my losses and got things working on ubuntu 18.04 first.

Someone might want to take a stab at getting this working on alpine, but I'd recommend it be another PR :)

Quick screenshare showing how this works (small typo, oops): https://recordit.co/SjnT1Oq40l

This will be essential on the way to #93

[patcon]

Here's my nonfunctional dockerfile for alpine linux in case anyone else wants to continue with it later: https://gist.github.com/patcon/78f53bf37dc954a4d238ad28f0789b1b

[mifi]

Awesome! Thanks