chore(docker): keep the build toolchain out of the runtime image and drop root

[idapixl]

The last two items from the #58/#60 audit backlog.

1. The C++ toolchain shipped to production

The runtime stage installed python3, make and g++ so npm ci --omit=dev could rebuild better-sqlite3 — and never removed them. Every published image carried a full compiler toolchain.

Purging them in a later RUN would not have fixed it: image layers are additive, so the packages stay inside the earlier layer even after removal. The only real fix is never installing them in that stage.

Production dependencies now compile in their own prod-deps stage, and runtime copies the finished node_modules. Both stages share node:24-slim, so the compiled native binary matches.

2. The container ran as root

Now drops to the unprivileged node user (uid 1000) the base image already ships.

The chown is load-bearing, not decoration. The SQLite store defaults to a relative ./cortex.db (core/config.ts), which resolves to /app at runtime. Everything copied in is owned by root, so dropping to node without chowning /app would turn the default configuration into a startup failure — the exact way a non-root switch usually regresses.

Verification status — please read

• Build: verified by CI. docker-publish.yml runs build-and-push on PRs with push: false, so this PR proves the image builds.
• Runtime: not verified. Docker was unavailable in the environment this was prepared in.

CI proves it compiles, not that it runs. Before relying on this, smoke-test:

docker run --rm -p 8080:8080 <image>
# confirm it serves on 8080 and can create cortex.db as uid 1000

The failure mode to watch for is a permission error on cortex.db creation. If the deployment mounts a volume over /app, the mount's ownership wins over the image chown and will need --user 1000 or an explicit chown on the host path.

[Copilot]

🔵 Needs a closer look

It changes production container runtime permissions and filesystem ownership behavior (not runtime-smoke-tested per PR description), which warrants a human-verified run check before approval.

Pull request overview

Updates the container build to keep the compiler toolchain out of the shipped runtime image and to run the service as an unprivileged user, reducing the production image attack surface and aligning runtime permissions with the SQLite default DB path.

Changes:

• Split production dependency installation into a dedicated prod-deps stage that includes the native build toolchain, then copy the resulting node_modules into the runtime stage.
• Drop root privileges in the runtime stage by switching to the built-in node user after ensuring /app is writable for the default ./cortex.db location.

File summaries

File	Description
Dockerfile	Introduces a prod-deps stage for native addon compilation and switches the runtime stage to non-root execution with a writable /app.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

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