I think you've misunderstood
Ok, OMV needs a separate (small) boot drive to install on (ie consider a M.2 / SSD on a USB adapter)
But, then all your (large) storage is used for the NAS.
OMV will run Docker containers, but their data would also be pointed to the large NAS storage.
| Small | Large |
|--------+-----------|
| OMV | Your Files|
| Docker | Data, etc |
Ah, ok, good to know, thanks