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)
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