The frontend Dockerfile ran only yarn build, so the container build
failed only on a broken build, not on lint or fmt-check regressions --
while script/cibuild's comment already assumed the Dockerfile ran make
check. Install make in the build stage and run make check (test + lint
+ fmt-check) in place of the bare yarn build. Its test step is the
production yarn build, so dist/ is still produced in a single build with
no redundant rebuild, and CI now goes red on any check failure.
