The build job will sometimes trigger a pull job when the base image
does not exist. Now that engine jobs properly close their output by default
the pull job would also close the build job's stdout in a cascading close
upon completion of the pull.
This patch corrects this by wrapping the `pull` job's stdout with a
nopCloseWriter which will not close the stdout of the `build` job.
Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Upstream-commit: e662775ffb096fdb4a7f247bb25b8e3022006c1b
Component: engine
3c4927bcb2