Create a non-root user 'ci' (UID 1000) in the Dagger toolchain container,
transfer ownership of the Flutter SDK and Android SDK to that user, and
switch to it with WithUser("ci"). Update all cache mount paths from /root/
to /home/ci/ and set Owner: "ci" on every WithDirectory call so Flutter
can write build output. Flutter emits a strong warning when run as root;
this change eliminates that warning by running the tool as a regular user.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
