util: Update GCN dockerfile for python3

util: Update GCN dockerfile for python3

This patch installs python3 in the GCN dockerfile, due to python3
being required starting in 20.2.

Python2 (python-yaml) is still required in order to compile rocBLAS

Change-Id: I1a6f1707e076e8ca499804119447a8d1f237ffd4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36158
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>