tests/docker: Use --userns=keep-id for podman

The workaround that attempts to accomplish the same result as --userns=keep-id
does not appear to work well with UIDs much above 1000 (like mine, which is
above 20000.)

Since we have official support for this "trick" now, use the supported method.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20190904232451.26466-1-jsnow@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

tests/docker/docker.py

@@ -369,9 +369,7 @@ def run(self, args, argv):
             argv = [ "-u", str(uid) ] + argv
             docker = Docker()
             if docker._command[0] == "podman":
-                argv = [ "--uidmap", "%d:0:1" % uid,
-                         "--uidmap", "0:1:%d" % uid,
-                         "--uidmap", "%d:%d:64536" % (uid + 1, uid + 1)] + argv
+                argv.insert(0, '--userns=keep-id')
         return Docker().run(argv, args.keep, quiet=args.quiet)