build: install corepack in the docker build stage - #3616

Summary

The Docker image build has been failing on main since the Node 26 upgrade (#3588): RUN yarn install exits with /bin/sh: yarn: not found (exit code 127) — failed run. The official node images stopped bundling Yarn v1 from Node 26 (nodejs/docker-node#2474), and Node itself stopped shipping Corepack from v25, so the build stage has no way to start Yarn. CI didn't catch it because the GitHub runners have Yarn installed globally; only the Docker build runs inside the bare image.

This installs Corepack in the build stage and enables it. Corepack resolves [email protected] from packageManager, which then defers to the vendored release via yarnPath. The Corepack version is pinned as a devDependency (0.36.0, past the 7-day npmMinimalAgeGate) and the Dockerfile reads it from package.json, so the existing npm Dependabot ecosystem bumps it — Dependabot's docker ecosystem only updates FROM tags, not version values in ARG/ENV. The production stage runs node directly and is unchanged.

Changes

• Dockerfile: after copying package.json, npm install --global corepack@<devDependencies.corepack> && corepack enable in the base stage, before any yarn call.
• package.json / yarn.lock: add [email protected] to devDependencies.

🤖 Generated with Claude Code