The scheduled builds are run from cron via `bash -l` - This is done in order to source the environment of the gitlab user.
When adding users with `--disabled-login` you will get `/bin/false` as your shell which breaks `bash -l`, meaning that unless `bundler` exists in `/usr/bin` (i.e. exists in one of the default PATHs) calling bundler will fail with `/bin/bash: bundler not found`.