Description of problem:

Running mock in a docker container, as root, fails with an error message related to the mock group not existing

Version-Release number of selected component (if applicable):

mock-1.4.16-2.fc30.noarch
mock-core-configs-31.1-1.fc30.noarch

How reproducible:

Always

Steps to Reproduce:
1. docker run -ti fedora:latest /bin/bash
2. inside the docker container, run "dnf -y install mock git", and wait for it to complete
3. run "getent group mock"
4. run "mock", without arguments (or with arguments, it doesn't matter)

Actual results:

"getent group mock" produces no output. "mock" produces the following output:

ERROR: "getgrnam(): name not found: 'mock'"
Traceback (most recent call last):
  File "/usr/libexec/mock/mock", line 977, in <module>
    main()
  File "/usr/lib/python3.7/site-packages/mockbuild/trace_decorator.py", line 96, in trace
    result = func(*args, **kw)
  File "/usr/libexec/mock/mock", line 644, in main
    mockgid = grp.getgrnam('mock').gr_gid
KeyError: "getgrnam(): name not found: 'mock'"


Expected results:

Either the mock group should exist after installing the package, or mock should be able to continue without it.

Additional info: