Updates the Dockerfile, and therefore the Docker image, to use Ubuntu 22.04 instead of Ubuntu 20.04. Change-Id: I0969205159d47ece9b0b6c0452ce0f3420f32e4c Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/69317 Maintainer: Bobby Bruce <bbruce@ucdavis.edu> Tested-by: kokoro <noreply+kokoro@google.com> Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>