This attempts to make it clearer that the --cgroup-parent option is only used
for the containers used during build. Instead of mentioning "build container",
I opted for using "RUN instructions" (to match the --network description),
although this may not be ideal (as it assumes the "Dockerfile" front-end, which
of course may not be the case).
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
