I have some other container that works with its user (with UID 1001) in the root group, GID 0.

When trying to set GID=0 for your ftp container, it first tries to create a group with -g 0 and name $FTP_USER, which fails as a group with that GID already exists with the name root.

However, in the next step, it tries to assign a group named $FTP_USER to the user. Since the previous step failed, there is no group with that name and the creation of the user also fails.

Please add something like a lookup for the group name, e.g.:

GRP = $(getent group $GID | awk -F ':' '{print $1}')

You can then assign the group name $GRP to the user creation command.