util: Updated the Dockerfiles for Python3
For the next release of gem5, we are dropping support for Python2. The
Ubuntu 18.04 Docker images were running with Python2. This has been
updated.
It should be noted that there is, at present, no eligant solution to the
issue that older versions of Scons (such as that obtainable via APT in
Ubuntu 18.04) use Python2. Those wishing to compile with these Docker
Images should use
`/usr/bin/env python3 $(which scons) build/X86/gem5.op5`
Change-Id: Ic36ecc7196688daff21af2bb3a76381966f38f60
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36595
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>