Install ESLint and prettier with yarn 4 through corepack (closes #493)

yarn 1, which the node image ships and which is no longer developed,
printed node's url.parse() deprecation warning during the js-deps
install. package.json now pins yarn 4.18.1 by version and hash in its
packageManager field; the js-deps stage enables it with the node
image's own corepack and runs `yarn install --immutable`. yarn.lock is
regenerated in yarn 4's format from the old lockfile, so every package
keeps the version it had. The new .yarnrc.yml keeps the install in
node_modules/, where the lint and Markdown stages run the tools from.

Model: opus-5-5

Dockerfile

@@ -67,15 +67,20 @@ RUN sed 's/}/}\n/g' static/css/tailwind.css > /tmp/committed.css \
 
 # JavaScript lint stages: ESLint, at the version package.json and yarn.lock
 # pin, checks static/js/ against eslint.config.mjs. js-deps installs it, and
-# prettier for the Markdown stages below, and stays cached until those two
-# files change. script/lint forces only js-lint to re-run, and the build stage
-# below runs it too. COPY . . brings in the CI cache barrier described in the
-# lint stage above.
-# node:24.21.0-alpine (LTS, with yarn 1.22.22), 2026-09-18
+# prettier for the Markdown stages below, and stays cached until package.json,
+# yarn.lock or .yarnrc.yml changes. script/lint forces only js-lint to re-run,
+# and the build stage below runs it too. COPY . . brings in the CI cache
+# barrier described in the lint stage above.
+#
+# The image's own corepack runs the yarn that package.json's packageManager
+# field names, yarn 4.18.1 (released 2026-09-24), and checks it against the
+# hash there. The image also ships yarn 1, which `corepack enable yarn`
+# replaces.
+# node:24.21.0-alpine (LTS), 2026-09-18
 FROM node:24.21.0-alpine@sha256:ebfe2f90462722a7a4de65e91990e97fe0d401c70e0e762c5b53302f905ec1c1 AS js-deps
 WORKDIR /src
-COPY package.json yarn.lock ./
-RUN yarn install --frozen-lockfile --ignore-scripts
+COPY package.json yarn.lock .yarnrc.yml ./
+RUN corepack enable yarn && yarn install --immutable --mode=skip-build
 
 FROM js-deps AS js-lint
 COPY . .