Testing PR #37358 on powerpc CI revealed a real bug (most probably unrelated to the PR).

CI logs show (from https://jenkins.dockerproject.org/job/Docker-PRs-powerpc/11035/console):

01:46:49.181 FAIL: docker_cli_daemon_test.go:1770: DockerDaemonSuite.TestDaemonNoSpaceLeftOnDeviceError
...
01:46:49.182 ... Error: Error while stopping the daemon d51b1c53d414d : exit status 2

daemon logs show:

panic: send on closed channel

goroutine 167 [running]:
github.com/docker/docker/pkg/progress.chanOutput.WriteProgress(0xc0000a57a0, 0xc001238967, 0xc, > 0x0, 0x0, 0x104987cd9, 0xa, 0x2b7b5c9, 0x2b7b5c9, 0x0, ...)
/go/src/github.com/docker/docker/pkg/progress/progress.go:42 +0x98
github.com/docker/docker/pkg/progress.(*Reader).updateProgress(0xc0000a3440, 0xc001c7e001)
/go/src/github.com/docker/docker/pkg/progress/progressreader.go:64 +0x130

Full log in the next message