README: Use --security-opt label=disable for SELinux systems
On SELinux systems using Podman as docker the container for testing can't
access files on the host by default. Disable the security checks so the
testsuite can run and dump screenshots.
On SELinux systems using Podman as docker the container for testing can't
access files on the host by default. Disable the security checks so the
testsuite can run and dump screenshots.