It seems that docker(?) doesn't like to be on the such kind of volumes made by mergefs or unionfs.
Perhaps it is good to be on the documentation as warning.
That's actually already in the documentation, you can read it in the warning box at this point. https://wiki.omv-extras.org/do…cker_in_omv#configuration
I realize that this document may seem a bit long the first time you read it, but I recommend reading it in its entirety at least once.
For mmap to be supported with mergerfs it must be enabled in current releases. Many users of sqlite3 force it to work with mmap despite not needing to and do a poor job of handling the errors. "No such device" is the low level error. You can read more in the mergerfs docs.
Actually I was reading the openmediavault-compose documentation, not the docker on OMV. Easy to miss this information. Perhaps a bit spread information between pages.
Docker is a fairly extensive topic. The Compose plugin isn't a plugin like openmediavault-symlinks, which can be explained in two lines. In fact, even if you read everything written on the wiki, you'll still miss explanations about some buttons that are added to the plugin's GUI over time. So, it's logical that there's more information, and that's why there are two documents on the wiki that complement each other. The Compose plugin offers links to both wiki documents in the OMV GUI.
Regarding the details you might miss, I have to say that unfortunately I don't have the time to keep both documents up to date. I'd like that to be the case, but my time on the project is limited. I'd like to take this opportunity to say that any help is welcome. In any case, I think I can say that the essentials for getting started with the Compose plugin are written there.
Using Docker requires a slight learning curve. The wiki documents cover that curve for use in OMV. If you want to delve deeper into Docker, there's still plenty of room to expand your knowledge if you read the official Docker documentation.
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