chore: accelerate docker compose by skipping frontend build

[mistercrunch]

while waiting for a docker-compose build, I decided to go and skip the long steps around the frontend buildsthat are not required while in dev mode while using docker-compose where we mount the local repo and build it inside docker.

Note that it's always tricky to have conditional logic in docker files as things aren't procedural. I took the approach of using and ARG and having conditions in RUN commands which accomplishes the goal of skipping the frontend builds in DEV_MODE=true.

changes:
- skip npm ci when DEV_MODE=true
- skip npm run build when DEV_MODE=true
- introduce an ARG for INCLUDE_FIREFOX, skip by default
- introduce an ARG for INCLUDE_CHROMIUM, include in dev layer by default - added a note in UPDATING.md even though people shouldn't really use the dev target from our Dockerfile.

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[rusackas]

Can we add some docs to the installing section that explains how/why one would change modes, and what does/doesn't happen in each mode? This is a fantastic optimization, but might lead to Github Issues if it's non-obvious.

One of these days, I'm gonna try switching us to Yarn. It'd be faster, and likely more stable, but would undoubtedly cause a lot of outcry/confusion :P

[mistercrunch]

@rusackas added some docs and rebased

[rusackas]

Just FYI, the link checker is non-blocking. I'm wondering if these guys went out of business or something. The site's been down for about a week thus far, and I didn't get a response from them on Slack.

[rusackas]

Love it! Thanks for the docs!