flaky test: TestPushWithContentTrustExistingTag

[thaJeztah]

seen failing sometimes: https://ci.docker.com/public/blue/organizations/jenkins/cli/detail/PR-2251/4/pipeline

=== FAIL: e2e/image TestPushWithContentTrustExistingTag (0.07s)
 push_test.go:294: assertion failed:
 Command: docker pull registry:5000/alpine:3.6
 ExitCode: 1
 Error: exit status 1
 Stdout:
 Stderr: error during connect: Post http://docker/v1.40/images/create?fromImage=registry%3A5000%2Falpine&tag=3.6: command [ssh -o ControlMaster=auto -o ControlPath=/root/.docker/%r@%h:%p -l penguin 172.20.0.4 -- docker system dial-stdio] has exited with exit status 255, please make sure the URL is valid, and Docker 18.09 or later is installed on the remote host: stderr=


 Failures:
 ExitCode was 1 expected 0
 Expected no error