Hi @oivanenko,
It is a normal behavior due to how Docker uses btrfs subvolumes for containers layers. As explained by @phillxnet in this post, these are detected by Rockstor as snapshots so deleting them actually corresponds to deleting the different layers of the containers. This is a known shortcoming and there already has been discussion on how to improve Rockstor’s behavior to this regard:
rock-ons-root snapshot warning · Issue #1682 · rockstor/rockstor-core · GitHub
I am actually interested in having a go at working on this as soon as I’m done with my current focus. In the meantime, however, you will see that in the first forum thread I referenced, there is an easy fix as you simply need to uninstall / reinstall the rock-ons in question. Using the same shares you defined for each will allow you to come back to where you were before without loosing any data (pending you chose dedicated shares for each as recommended).
Let us know how it goes 