Build the dev container in CI only when its inputs change
The test-devcontainer job built the container from scratch on every pull request, about 10 minutes. It now lists the pull request's files first and builds only when the container definition, the opam or yarn dependencies its setup installs, the Makefile or ci.yml change; pushes, and pull requests whose files can't be listed, still build it. The job always runs, so ci-ok still sees it succeed.