util-docker: Use python3 by default for Ubuntu 18.04 docker images

gem5 dropped the support for python2. This change sets python3
to have a higher priority than python2 in the Ubuntu 18.04 docker
images. This is done so that gem5 will be compiled and tested
in the python3 environment by default.

JIRA: https://gem5.atlassian.net/browse/GEM5-958

Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Change-Id: I11ffb06697ecf4cebf9f98b611641faa42805547
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45125
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45826
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
2 files changed