The configured Ubuntu 24 base image already contains an ubuntu user and group, so .devcontainer/Dockerfile fails when the host's USER is ubuntu. Reuse existing accounts and groups, preserving their IDs and home directories, and assign newly created users to their matching named group. Dev Containers continues to handle host UID/GID alignment on Linux.

Add .devcontainer/user-setup-test.sh to exercise existing users, new users, and existing groups against the Dockerfile's exact base image.

Validation: all three Docker regression scenarios pass; shell syntax and whitespace checks pass. bazel run format passes for both changed files with --//bazel/config:sdkroot=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk. The full dev-container image build has not been run.