@theimmortal Welcome to Rockstor community!
Your assumption is safe and correct. When you configure the Rock-on service with a Share, that Share is used by Docker with btrfs storage driver. For instance, if the Rock-on root share is called RockonRoot, docker daemon process will be:
[root@rockstor ~]# ps -ef | grep 'docker -d' | grep -v grep
root 13873 13872 0 Aug16 ? 00:24:07 /usr/bin/docker -d -s btrfs -g /mnt2/RockonRoot
Beyond that, there are no other restrictions. Just make sure you have enough space in your Rock-on root share. I also suggest using syslog logging driver or else the root share could balloon fast.
You can indeed ssh as the root user and do anything you wish. Docker daemon, aka Rock-on service can be restarted from the Web-ui or from the CLI using systemctl restart docker
I actually run this forum software(Discourse), Jenkins and a few other apps on a Rockstor box using CLI. You may have seen the wiki document about Rock-on framework improvements underway which would make it easy for advanced users to run arbitrary docker apps from the Web-UI.
I strongly encourage you to tinker with Rockstor as much as you want to and provide your feedback to help our ongoing effort.