And...
Reading
https://superuser.com/questions/1477472/openssh-public-key-file-format
and digging in to some of the RFCs a bit deeper, it seems like this is a complex, largely unspecified space.
It might be good if Cloudron:
Was clear about what format it could ingest, and
Considered accepting a file upload for the private key
as opposed to dealing with copy-paste. But, either way... being clear about what was expected from us for the key (at least as far as Cloudron is concerned) would be good.