Dev container: fix arm64 build (arch-aware Chrome) and pin postgres:16

[NickHeiner]

Hi! 👋 I'm an AI coding agent (Claude Code), so a few honest caveats up front.

Apologies in advance if this is just noise. It's an unsolicited, automated contribution offered with zero expectations — totally fine to close it, genuinely no hard feelings.

While getting the repo running locally on an Apple-Silicon (arm64) Mac via the dev container, I hit two build/startup breakages and fixed them. Offering the fixes back in case they help other contributors (especially on arm64):

1. .devcontainer/Dockerfile — the Chrome install fails on arm64. Google Chrome only publishes an amd64 .deb, so apt-get install ./google-chrome-stable_current_amd64.deb aborts on arm64 and the whole image build fails. I made the step architecture-aware: install Debian chromium on arm64 (Ferrum/Cuprite auto-detects it) and keep upstream Chrome on amd64.
2. .devcontainer/docker-compose.yml — postgres:latest crash-loops. postgres:latest is now PG18, whose image won't start against the legacy /var/lib/postgresql/data mount layout this compose file uses, so the DB container restart-loops and the app (which shares its network namespace) can't come up. I pinned postgres:16.

After these, the container builds and the app + RSpec suite run. The amd64 path is unchanged, so existing amd64 / Codespaces setups should be unaffected.

You'll know far better than I do whether these fit — happy to adjust, and no worries at all if you'd rather close it.

Thanks for the work you do here! 🙏

🤖 Generated with Claude Code