See it this way:
Docker makes sure that nextcloud and OMV are different from each other and that you know where to look for the bug.
henfri, if you want to talk about the bad side of docker then that's easy to state a working example as per it's relation to openmediavault ....
SIZE, as you have no option but to install hundreds of megabytes of redundant code on an obscure root/boot only PATH (NOT EVEN INSTALLED ON THE ACCEPTED official "opt", AS IN OPTIONAL linux relative path" just to run the simplest of applications and / or scripting web language on a generic openmediavault that makes a point of actually running fine on a tiny usb3 stick and the like (diy 30gig M2 ssd in a usb3 enclosure etc).
as an extreme example see multi-platform "rebol" the web scripting app in a few hundred bytes
https://hub.docker.com/r/sthysel/rebolcore/ and compare size and actual execution speed with any fully working shell/GUI rebol html script you like.
plus OC open mediavault still does not have a workable docker absolute/relative path GUI (unless you are an expert in the shell initiation and linux execution rights to actually see one real "shared folders" whatever) share its root 911 or whatever user id it wants to use as the generic default user setting etc ..., you get the point ...