website: Added Ubuntu 20.04 Docker image link to building doc

Change-Id: Ie711209fa993e4270dd824558fdd75abfa527e1e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/28869
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
1 file changed