docs: document the docker workflow

[Aymericr]

Docker support landed in #268, but nothing in SETUP.md or README.md mentions it — the only way to discover it was to notice docker-compose.yml in the file listing.

Adds a short section with the one command, where saved scenes live, and the port constraint. That last one is worth writing down because the failure is non-obvious: remap the host port and /scenes returns 500, since the page derives its own API base URL from a value Next inlines at build time.

Docs only.

🤖 Generated with Claude Code

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Note

Low Risk
Documentation-only change with no runtime or application code impact.

Overview
SETUP.md now documents how to run the editor with Docker after support landed elsewhere without setup instructions.

The new section covers docker compose up -d, http://localhost:3000, and persistence via the pascal-data volume across docker compose down. It also warns not to remap the container port: /scenes builds its API base from NEXT_PUBLIC_APP_URL, which Next inlines at build time, so a different host port can surface as a non-obvious 500 on that page.

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