Welcome to the forum @narflar
I don’t use Docker, but I’ll point to How to log a backup job to stdout? where lots of experimenting was done.
That error message looks like you might have left off the command for Duplicati, and tried to “run” an option.
https://github.com/duplicati/duplicati/blob/master/Installer/Docker/context/Dockerfile might be the Dockerfile.
krallin/tini: A tiny but valid init for containers looks like the ENTRYPOINT, and I think CMD is appended to it.
It’s been some years, and things might have changed, but you could see if this old test still gives any output:
If so, then that’s one part of the puzzle solved. The other parts include getting duplicati-server to write stdout. Telling it to append to --log-file=/dev/stdout might get you a seek error. It probably wants a normal file to write.
The other question is – what do you want written? The server normally interacts with the user over HTTP so status and such are given that way. The CLI client is designed for stdout, but it’s not going to work as server.
Duplicati.Server.exe shows its options (also available in its help command). I think it ignores unknown ones.
--log-file: Outputs log information to the file given
--log-level: Determines the amount of information written in the log file
are probably the ones to play with. For --log-file, the goal is finding something Duplicati will write and Docker will read. For --log-level, Warning will get warnings and errors, but not much of the context CLI client writes. Information gives more, but is it too much (e.g. do you really want to log all the file uploads taking place?).
Reporting options shows lots of other ways to get reports, and they can be fed into several monitoring tools, should you want more features than the raw feed from Duplicati. To get live log, Duplicati has one in its GUI.
duplicati-client goes into the Duplicati server interface from a command-line client if you’re seeking scripting.