Fix CI: ignore hadolint DL3066 for the named user in the Dockerfile

[felladrin]

Description

Currently, CI is red on main, and it blocks every open PR from going green. The Lint Dockerfile step fails with Dockerfile:43 DL3066 info: Non-numeric user-id may not be resolvable by host system, pointing at USER ${USERNAME}.

This PR adds DL3066 to the ignore list in .hadolint.yaml. The image runs as the named node user that the node base image already provides, so the non-numeric user-id resolves inside the container. That's by design, not something to fix in the Dockerfile.

Root cause

#2311 bumped hadolint/hadolint-action to v3.4.0, which pins hadolint:v2.15.0-debian. That version reports DL3066 where the previous one did not. The action's default failure-threshold is info, so an info-level finding is enough to fail the job. #2310, the commit right before it, has a green run, which confirms the bump is what changed.

The step also short-circuits the rest of the job: Check formatting and Run tests are skipped when the Dockerfile lint fails, so no PR gets its tests run in CI until this lands.

Why the ignore, and not a higher threshold

Raising failure-threshold above info would also fix the red build, but it would stop info-level findings from blocking anything else, for a single rule we disagree with. The ignore entry is narrower.

By the way, this rule looks like it was short-lived upstream: hadolint 2.15.1 no longer reports DL3066 on this Dockerfile, only 2.15.0 does. So a future action bump may make the entry unnecessary. It's harmless to keep either way, and I'd rather not wait on a release we don't control.

How to test

1. Check out the branch and run hadolint with the exact image the action pins:

docker run --rm -i -v "$PWD/.hadolint.yaml":/.hadolint.yaml ghcr.io/hadolint/hadolint:v2.15.0-debian hadolint --config /.hadolint.yaml --failure-threshold info - < Dockerfile

It exits 0. On main the same command reports DL3066 and exits 1.

2. Confirm the ignore list is still narrow (other rules keep firing at the same threshold):

printf 'FROM node:lts\nRUN cd /tmp\n' | docker run --rm -i -v "$PWD/.hadolint.yaml":/.hadolint.yaml ghcr.io/hadolint/hadolint:v2.15.0-debian hadolint --config /.hadolint.yaml --failure-threshold info -

It reports DL3003 and SC2164, and exits 1.

3. Run the two steps that were being skipped: npm run format:check passes (172 files, no fixes applied), and npm run test:coverage passes (40 test files, 349 tests).

Note: npm run lint exits 1 on my machine, on main too, from a knip "Unlisted binaries: playwright" report. It's a local environment difference, not from this change; the Run lint step passed in CI on the failing run.