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   | Files         |
| Docker| Data, etc |