misc: Added Dockerfile for GCC of different versions

misc: Added Dockerfile for GCC of different versions

This will create a Docker image with all gem5 dependencies, allowing for
a specfic GCC version to be specified via `--build-arg version=X`. I.e.,
to create an image with GCC 8,
`docker build util/dockerfiles/gcc-version --build-arg version=8`.

Issue-on: https://gem5.atlassian.net/browse/GEM5-228
Change-Id: I927eb90b6446059cce70e3b722a8fc3985068285
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26507
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>