feat: add a few docker-compose env vars to control the build + docs

[mistercrunch]

Living in docker/compose over the past few days, I figure it'll be good to attach build
args to env vars so it's easy to turn things on and off.

As part of this, I'm turning off installing chromium in the build as in most cases developers
won't be working on something that requires it. If you need though, it's
easy to set an env var to switch it on.

Should speed up the local builds a little and make images significantly
smaller locally.

[rusackas]

I'm not sure if this qualifies as "breaking" for certain edge cases, but it could be added to the board if it seems like we ought to play it safe. Today is the deadline for such proposals.

[mistercrunch]

Oh I think the bulk of the changes here got merged in my other PR here. I kept the default in Dockerfile and set new aligned defaults in docker-compose.yml .

The main change that may affect people is that Chromium is no longer in the dev docker image, but only for docker compose. Adding it back if/when needed is now well documented.

The benefit is we all get leaner dev environments that build/load faster.

I could add a note in UPDATING.md but that shouldn't affect docker images, only the dev/docker-compose context

[sadpandajoe]

@mistercrunch looks like there are some failures when pushing docker in master looking at the master commits. Not sure why they aren't failing in branches but wonder if we could catch these things before they merge

[mistercrunch]

Yeah I saw, will fix tomorrow, issue is around the fact that we --load now for docker-compose to run test, and that --load is not allowed for multi-platform builds, and it's set up to do multi-platform builds on master and single-platform on PRs (explains why I didn't catch it on my PR's CI). I'll just have to not --load and add a step to docker pull in CI