I’m using a Windows system that does not already have SSH fingerprint. If you have that, try deleting it, however if you have none too, and it says nothing on Test connection or backup, that seems strange.
Have you looked in Destination screen Advanced options or Export As Command-line for fingerprint?
How does MD5 (obsolete and insecure) enter in, and what does “pulled” mean? This isn’t a file hash.
If you mean Duplicati Docker, which one (Duplicati or someone else’s). Or is sshd in a container?
Regardless, I don’t use Docker, so I can’t get into details, but don’t keep data in the container itself.
EDIT:
In addition to (and in basic agreement with) the earlier link I posted, here’s a recipe for getting fingerprints:
About the SSH host key fingerprint
Although I doubt it’s the case, if Duplicati Docker comes with a preinstalled ssh-fingeprint, that’d be a bug. Docker for the sshd side seems potentially likely to use the same /etc/ssh files, so may be prone to this…