docker image to use non-root user

[rijobro]

Is your feature request related to a problem? Please describe.
Our cluster has disabled the user, root. This then means that using MONAI's docker image causes error since it only has one user, root. This problem may also affect other users, where they can't use root for various reasons. For example, I think by default, you can't SSH into a root user.

Describe the solution you'd like
In the Dockerfile, I'd like for a new user to be added (e.g., monai) with sudo privileges and for this to be the default.

Something like (not tested):

RUN adduser --disabled-password --gecos '' monai
USER monai