I setup jenkins 2 and provided a -u option to the docker run command to set the user.
docker run --restart=always -d -p 80:8080 -p 50000:50000 -u `id -u jenkinsmaster` -v `pwd`:/var/jenkins_home jenkins:2.3

Then I configured my setup with a build slave over SSH.

Then I added a multibranch pipeline project.

The build slave failed to check out the git project with an error message:
No user exists for uid 1003.

1003 is the uid of jenkinsmaster (id -u jenkinsmaster). It is normal that there is no such user on the build slave. It is peculiar the build slave even knows about the build master user id and that this causes the git checkout to fail on the slave.

Note: docker is only used to run the jenkins master in this setup.