website: Added notes on using Docker to build and run

In the Building documentation, a section has been added outlining how a
Docker image may be obtained, and used, to run gem5.

Change-Id: Ic9ff026e47e21bb0aa1e11a32fc3df753affb991
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/28867
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
1 file changed