docker-compose.yml: Force testnode arch
docker-compose.yml: Force testnode arch

I initially worked on this on an M1 Mac, and that caused problems with
scheduling due to a lack of aarch64 packages. The M1 can run amd64
images, though.

Signed-off-by: Zack Cerza <d7cdf09fc0f0426e98c9978ee42da5d61fa54986@redhat.com>

ceph/teuthology,ktdreyer/teuthology,ktdreyer/teuthology,ceph/teuthology