If do not update the UID within the passwd database to match whatever
uid the container is being ran as, then sudo will not work when renaming
the user to match $DOCKER_USER as it will complain about the current
user being non-existent.
We do not try renaming $HOME anymore as there is no good way
to do it.
We also only try to convert if the user hasn't been changed.
Finally I added usage to the docker docs in install.md
Closes#2056
