[regression in v18.06] Unknown top-level command should fail with non-zero exit code

[AkihiroSuda]

Description

Unknown top-level command should fail with non-zero exit code

Steps to reproduce the issue:

$ docker foobar
$ exit $?

Describe the results you received:

0

Describe the results you expected:

1

Output of docker version:

Client:
 Version:           19.03.0-dev
 API version:       1.39
 Go version:        go1.10.4
 Git commit:        ab50c2f2
 Built:             Wed Oct 10 08:56:03 2018
 OS/Arch:           linux/amd64
 Experimental:      true
...

[AkihiroSuda]

This is regression in v18.06

$ docker run --rm docker:18.05 docker foobar;  echo $?
docker: 'foobar' is not a docker command.
See 'docker --help'
1

$ docker run --rm docker:18.06 docker foobar; echo $?
                                                                             
Usage:  docker [OPTIONS] COMMAND
...
Run 'docker COMMAND --help' for more information on a command.
0

[AkihiroSuda]

regression in a3fe7d6

cc @thaJeztah ^^

[AkihiroSuda]

PR: #1429