docker: let _copy_with_mkdir() sub_path argument be optional

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

tests/docker/docker.py

@@ -52,7 +52,7 @@ def _guess_docker_command():
     raise Exception("Cannot find working docker command. Tried:\n%s" % \
                     commands_txt)
 
-def _copy_with_mkdir(src, root_dir, sub_path):
+def _copy_with_mkdir(src, root_dir, sub_path='.'):
     """Copy src into root_dir, creating sub_path as needed."""
     dest_dir = os.path.normpath("%s/%s" % (root_dir, sub_path))
     try: