It mirrors the existing DOCKER_CMD, both in how it's defined
as part of the environment and how it's called by passing it
directly to the shell.

In addition to making the configuration more consistent, this
also allows us to move from having the macOS build script
divided into four steps, some of which have slightly different
semantics and the relationship between which is not immediately
obvious without consulting the documentation, to a single
straightforward shell invocation.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>