Fix help text being incorrect with multiple args

E.g. "docker foobar run" would have printed "Command not found:
foobar" and printed the help text for "run". It should instead
print the root help message for docker.

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Merge pull request #8927 from bfirsh/dont-pass-args-to-help-when-command-doesnt-exist

Fix help text being incorrect with multiple args