The 'build images' workflow runs on a schedule, so will run (and fail)
weekly in every fork.
Don't try to push to docker if docker credentials aren't in repository
secrets.
(A test for that has to be written rather indirectly due to
https://github.com/actions/runner/issues/520)
D lang compilers have an option -release (or similar) which turns off
asserts, contracts, and other runtime type checking. This patch wires
that up to the b_ndebug flag.
Fixes#7082