I don’t know what’s in your Docker container, but just letting it sit isn’t adding much debug data.
Taking resource (e.g. memory) measurements on the container might be good to rule those out.
Possibly netstat -a or similar would show connections to 1drv.ms, and give connection state.
There’s also a mono thread dump ability, but I’m not sure if you can easily catch the output data.
Getting Duplicati to run in a terminal would. I suspect Docker won’t, but I don’t have any Docker.
I also don’t have much Linux (just some small VMs on Windows). Do you see this on Windows?
Attempting to simulate normal backup. You posted that you saw this with a “regular backup” too.
OK, next test is backup 10 GB of data at 50 MB, then will try standalone compact to 250, as OP.
I see 3 GB, but dblock-size is unclear. dblock-size suggests 6 MB, backend values suggest 1 MB.
I can tune next run, but I’m still not particularly confident that this system will show any such hang.