fix: refactor Dockerfile to speed up run times

[jescalada]

Fixes Dockerfile slow runtimes (5-10 mins each) to speed up E2E test execution speeds.

Note that Docker Compose step execution times may vary considerably depending on whether we've executed the CI "recently" (perhaps within a few hours). Another factor might be Docker/NPM traffic spikes that make fetching slow.

I've seen worst-case executions of around 15 mins:

Before

~7m without caching

~2m 10s when cached

After

~4m 50s without caching (~30% faster)

~1m 20s when cached (~35% faster)

[netlify]

✅ Deploy Preview for endearing-brigadeiros-63f9d0 canceled.

Name	Link
🔨 Latest commit	d3cc065
🔍 Latest deploy log	https://app.netlify.com/projects/endearing-brigadeiros-63f9d0/deploys/6a0ab44854e66200083577b2

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.25%. Comparing base (4c6e8d4) to head (d3cc065).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1521   +/-   ##
=======================================
  Coverage   90.25%   90.25%           
=======================================
  Files          69       69           
  Lines        5561     5561           
  Branches      958      958           
=======================================
  Hits         5019     5019           
  Misses        523      523           
  Partials       19       19           

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