Facilitate copying files between a host and running container

All commands are run as sudo docker exec or podman exec:

Line 319 in 7defa70

code, stdout, stderr = self.machine[args[0]].run(args[1:], **kwargs)

We should implement utilities for copying content between a host and container. It is useful when dealing with temporary directories/files that were created on the fly and should be present on the Pulp's instance too (e.g., GnuPG home directories). It will eventually allow us to test features used by administrators (where direct access to Pulp is needed).

https://docs.docker.com/engine/reference/commandline/cp/

https://docs.podman.io/en/latest/markdown/podman-cp.1.html

This can now be closed because we are running functional tests from within a container. Copying files between the host and container is no longer needed. Temporary directories (e.g., GnuPG home directories) created in the functional tests' user space are accessible by all parties: https://github.com/pulp/pulp_container/pull/735/files#diff-6d1431d4ae17b8ee6c68c9e26883fe1fe84425cbc7f92c15a9e4683fc0c174aaR152-R221.