adduser shows uid '200' in use when the gid is in use
adduser
incorrectly reports the uid 200 to be already in use - the actual problem is that it tries to create a group with gid 200 and the gid is already in use.
The error message should read adduser: gid '200' in use
or maybe even adduser: gid '200' in use by group nofiles
to make it clear what the problem is.
$ docker run --rm -it alpine sh
/ # adduser -u 200 test
adduser: uid '200' in use
/ # grep 200 /etc/passwd /etc/group
/etc/group:nofiles:x:200:
/ # adduser -u 200 -G nofiles -D test