build(afm): run dev/CI containers as a non-root user

[P4suta]

What

The dev image ran as root, so every file a container wrote into the
/workspace bind mount (generated artefacts, wasm pkg/, mdbook output,
node_modules) landed root-owned on the host — the classic bind-mount
footgun, and not how a dev container should behave.

This switches the dev/CI containers to a non-root user matched to the host
UID, the industry-standard pattern.

How

• Dockerfile (dev + book stages): create a non-root dev user
  (UID/GID default 1000, overridable via --build-arg UID=/GID=),
  chown the /cargo cache dirs and the playground node_modules
  mountpoint so fresh named volumes initialise dev-owned, and USER dev
  at the end of each stage. The fuzz stage flips back to USER root for
  its /usr/local installs (nightly + cargo-fuzz/udeps), then drops to
  dev again.
• docker-compose.yml: user: "${AFM_UID:-1000}:${AFM_GID:-1000}" on the
  dev/fuzz/ci/book/playground services. Unset locally → the
  host-matched 1000 (host-owned writes); CI sets AFM_UID=0 → root.
• setup-dev-image action + book CI job: emit AFM_UID=0/AFM_GID=0
  so CI stays byte-identical to the proven root behaviour — the
  ephemeral runner's checkout is owned by a different UID and a non-root
  container can't create files (pkg/, book/, dist/) inside it.

Rationale for keeping CI as root

CI is ephemeral and ownership is throwaway there; the runner's checkout is
owned by a different UID than any baked image user, so root is the
simplest correct choice and avoids a cross-UID write-permission class of
failure. Local dev — where the bind mount is the developer's real tree — is
where non-root matters, and that's now fixed.

Verification (local, UID 1000)

• docker compose run --rm dev id → uid=1000(dev); AFM_UID=0 … id → root.
• A container-written bind-mount file is yasunobu:yasunobu-owned (was root).
• Pre-existing root-owned cargo volumes migrated in place (chown -R 1000 /cargo, no cache loss); fresh playground-node-modules volume initialises dev-owned.
• just ci green non-root (pre-push gate); just book-build writes host-owned book/.

🤖 Generated with Claude Code