Adding zsh completion

[silvin-lubecki]

- What I did
Adding the zsh shell auto-completion scripts.

- How I did it

• As suggested here I was largely inspired by kubectl. Default zsh completion generated dynamically by cobra is not really appealing (it does not complete the flags), so as kubectl, I use the bash completion scripts with some bootstrap scripts.
• Bumped docker/cli to get a fix on long help messages

- How to verify it

$ source <(docker-app completion zsh)
$ docker-app <tab>
... it should display commands and flags...

- Description for the changelog

• Adding zsh completion
• Bumping docker/cli

- A picture of a cute animal (not mandatory but encouraged)

[chris-crone]

@silvin-lubecki please rebase this

[silvin-lubecki]

Yep I'm on it!

[vdemeester]

We should hold on to merging that (i.e. the docker/cli bump) until docker/cli#1361 is merged, it will reduce a lot the number of dependencies

[vdemeester]

@silvin-lubecki upstream PR merged, can you update vendors once again ? 👼

[silvin-lubecki]

PTAL @vdemeester

[vdemeester]

LGTM 🐸

[mnottale]

LGTM