uidmap with AD username format

Hi, we are using uidmap for our rootless docker installation. The format of our AD user is DOMAIN\USERNAME. When this format is added to the subuid and subgid file it does not recognise the username format.

rootless dokcer suggests this uidmap entry format for us:
DOMAINUSERNAME:100000:65536

But this does not work

The only way that we are able to get the subuid and gid requirements fullfilled for rootless docker is to have these 2 entries together:

DOMAINUSERNAME:100000:65536
DOMAIN\USERNAME:100000:65536

Any explanation for that?

Start by reproducing the uidmap behavior with rootless Docker and DOMAIN/USERNAME entries in the subuid and subgid files. Trace how the username formats are resolved and document or correct the behavior so the required configuration is clear and consistently recognized.

Written by the indexing model from the issue text.