Podman compose results user owned

[cedricbu]

Podman 3.1.0 and above support the "U" flag to change ownership of
volumes.
This makes the `unshare chown` unecessary when using podman >= 3.1
IIUC, it does not affect older versions, which will simply ignore the
unknown flag.

[lpardoRH]

LGTM but README file should be updated to remove the unshare directive

[cedricbu]

Oh, yes, good point, I forgot that. I updated it.

... Also, I see that the "Warns when uploading a policy that exists already" accidentally found its way into this branch :/ ....

[cedricbu]

:) all cleaned up.

• I removed the changes that I accidentally sneaked up ("Warns when uploading a policy that exists already", which belonged to another PR)
• modified the podman-compose files for automatically changing the ownership
• commented the README about the change

[lpardoRH]

LGTM @jeremychoi