Remove (theme)`Dockerfile`

[mattxwang]

trafficstars

Draft since I still need to spend a bit more time fleshing this out, but in short: it doesn't seem like any of our theme developers use this (out of date) dockerfile, and we're probably better served placing more of this in the theme template (so users can consume it). I think this is part of our overall movement to not have users of our theme directly clone this repo.