As per documentation, ADD would set the UID/GID to 0 on all files copied into the filesystem.

However, the UID/GID are preserved from the host.

From inside a container:
docker-container$ ls -l devstack
-rw-rw-r-- 1 107 113 57 Feb 3 18:10 devstack

From my build-host:
jenkins-slave$ id jenkins
uid=107(jenkins) gid=113(jenkins) groups=113(jenkins),999(docker)