wip: build(docker): build node app in a separate docker build stage DEV-1343

[p2edwards]

WIP

• perf: these layer revisions might improve docker build times on average
• maint: a step towards running frontend dev server in a different docker container than the django app

🗒️ Checklist

1. run linter locally
2. update developer docs (API, README, inline, etc.), if any
3. for user-facing doc changes create a Zulip thread at #Support Docs Updates, if any
4. draft PR with a title <type>(<scope>)<!>: <title> DEV-1234
5. assign yourself, tag PR: at least Front end and/or Back end or workflow
6. fill in the template below and delete template comments
7. review thyself: read the diff and repro the preview as written
8. open PR & confirm that CI passes & request reviewers, if needed
9. delete this section before merging

📣 Summary

TODO

📖 Description

TODO

👷 Description for instance maintainers

TODO

💭 Notes

TODO

👀 Preview steps

1. ℹ️ have an account and a project
2. do this
3. do that
4. 🔴 [on main] notice that this isn't anywhere
5. 🟢 [on PR] notice that this is here
6. do that another thing
7. 🟢 notice that this changed like that