Docker build taking way too long on Heroku

[metasoarous]

Right now we're deploying with Heroku (see #962 #1301), but finding that it's taking up to 40 min to build the docker images necessary to run using the docker infrastructure, which is insane. Ideally, we'd have a much tighter turn around.

A big improvement could probably come from taking the long ./Dockerfile at the root of the project (which builds all of the clients), and consolidating some of the build steps. This will get a lot better when we merge some of the clients (@colin mentioned wanting to work on this once we get the dev environment smoothed out), but for now, I'm wondering if we can create a shared apk context for the client building parts in that dockerfile, so that it's not having to run apk add ... over and over; We may be able to create a single intermediate image that inherets from node:*.*.*-alpine, and use that as the base for each of the client build phases.

We may be able to clean up some of the other docker pieces as well.