ci: Generalize running commands inside the container

ci: Generalize running commands inside the container

Both for ci-build and ci-shell we want to execute basically
the same setup and cleanup logic, the only difference being
that for the former we then run the build script and with the
latter a shell.

Rework the targets so that they both call the generic
ci-run-command rule passing an appropriate $(CI_COMMAND).

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>