ci: change docker root password to a non-empty string

[cuiy0006]

Set the docker root password to cloudcxx, because empty password doesn't work for su - in docker container.

---

This change is 

[dbolduc]

Thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.58%. Comparing base (e353a33) to head (efd2ba2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14470      +/-   ##
==========================================
- Coverage   93.59%   93.58%   -0.01%     
==========================================
  Files        2315     2315              
  Lines      207140   207140              
==========================================
- Hits       193869   193858      -11     
- Misses      13271    13282      +11     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.