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 <vincent@bernat.im>
Merge pull request #9291 from vincentbernat/fix/zsh-completion-update-2

zsh: correctly parse available subcommands (completion)