zsh: correctly parse available subcommands
zsh: correctly parse available subcommands

A lot of flags have been added on the output of `docker help`. Use a
more robust method to extract the list of available subcommands by
spotting the `Command:` line and the next blank line.

Signed-off-by: Vincent Bernat <87e1f221a672a14a323e57bb65eaea19d3ed3804@bernat.im>

riyazdf/cli,riyazdf/cli,Zebrilee/cli,redpanda/cli,cpuguy83/cli,thaJeztah/cli,dnephin/cli,thaJeztah/cli,ashfall/cli,docker/cli,docker/cli,e11137/cli,ashfall/cli,cpuguy83/cli,allencloud/cli,e11137/cli,Zebrilee/cli,dnephin/cli,redpanda/cli,allencloud/cli