The bin/solr script uses whoami to determine if it is running as root.
That code is here and was introduced with this commit for
This can produce a warning if the effective uid is not present in /etc/passwd.
$ docker run -it -u 88888 bash bash -c 'whoami' whoami: unknown uid 88888
This is an unusual situation, but one I encountered and worked around
The use of whoami can be avoided by using id -u, or better yet $UID / $EUID which is built into bash and thus also avoids a subshell invocation. I'm not aware of any downside to this approach.