Makefile: Use `?=` for initial variable definitions(part 2)

[anoopcs9]

Iteration #2 of #725.

CEPH_IMG and CEPH_TAG are commonly used to include custom ceph images as base while building go-ceph container images for testing and development purposes.

[phlogistonjohn]

I don't use the environment this heavily for make, but OK. In the meantime, wrt to the failing nautilus jobs do you prefer to have this PR first with an admin override of the requirements, or wait until we fix/resolve that and then come back to this topic?

[mergify[bot]]

This pull request now has conflicts with the target branch. Could you please resolve conflicts and force push the corrected changes? 🙏