misc: Added Dockerfile for Python3

misc: Added Dockerfile for Python3

This will create a docker image with all gem5 dependencies in a Python3
virtual environment.

Issue-on: https://gem5.atlassian.net/browse/GEM5-392
Change-Id: Id23777fb698977e92437c546f1fdf0ea0faa8708
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26506
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>