test: findUserArg has no unit tests

`findUserArg` in `singleContainer.ts` is exported but has no test coverage.
No test file currently imports from `singleContainer`.

Proposing to add `src/test/singleContainer.test.ts` covering these cases:
- ['-u', 'myuser'] -> 'myuser'
- ['--user', 'myuser'] -> 'myuser'
- ['--user=myuser'] -> 'myuser'
- ['--user', 'a', '-u', 'b'] -> 'b' (last wins)
- [] -> undefined

Would this be helpful?

Start by reading findUserArg in singleContainer.ts and reviewing nearby test conventions. Add src/test/singleContainer.test.ts for the five listed argument cases, then run that test file using the repository's existing test command. Done means each expected username or undefined result is covered and passes.

Written by the indexing model from the issue text.