Enhance reproducibility by setting deterministic hostname/resolver

As detailed in mmdebstrap(1), mmdebstrap copies the host’s /etc/resolv.conf and /etc/hostname into the chroot. This means that the mmdebstrap output will differ if it is run on different gitlab runners. To make the result reproducible across different hosts, we fill them with reproducible content.

Ref: team#41328 (closed)

note: this depends on !3 (merged) being merged note2: the MR CI build failures are related to team#41354 (closed) if this ran on the runners that are noted in that issue to succeed, the jobs would succeed.