Connect to docker container as user other than root

Question

BY default when you run

docker run -it [myimage]

OR

docker attach [mycontainer]

you connect to the terminal as r

Answer 1

The only way I am able to make it work is by:

docker run -it -e USER=$USER -v /etc/passwd:/etc/passwd -v `pwd`:/siem mono bash
su - magnus

So I have to both specify $USER environment variable as well a point the /etc/passwd file. In this way, I can compile in /siem folder and retain ownership of files there not as root.