Fix nsjail_test.py when run as root:root

Tweaked the comparison in nsjail.py to accept all integers including 0
for user_id and group_id. This allows the unit test to pass when run as
root:root. Now all unit tests can be run from Cloud Build.

Updated README.md to indicate dockerfile_test.py should be run from the
Cloud Build workflow.

Testing: Local, Cloud Build
Bug: 120137633

Change-Id: I3fe9f2e69d35f9c43e085b6ec30f46b5f673e8c9