misc: Hardcode docker pulls to v21-2 versions

Change-Id: I404f2c881a00645302c0af5f22e8a59548426cac
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/54470
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
4 files changed