Added docker context to zsh completion

[stoffus]

- What I did

Added zsh completion for the docker context subcommands.

- How I did it

Followed the convention of the other completions.

- How to verify it

1. Replace the content of the current _docker plugin (most likely located in ~/.oh-my-zsh/plugins/docker/_docker for Oh My Zsh users).
2. If not already present, append the docker plugin in your .zshrc, e.g. plugins=(git mvn docker).
3. Reload zsh with source ~/.zshrc.
4. The plugin should now include the new context subcommands, e.g. docker context use <tab>.

- Description for the changelog
Added zsh completion for docker context subcommands.

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

[codecov-io]

Codecov Report

Merging #2906 (b04241d) into master (2291f61) will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## master #2906 +/- ##
=======================================
Coverage 57.05% 57.05% =======================================
Files 297 297 Lines 18653 18653 =======================================
Hits 10643 10643 Misses 7151 7151 Partials 859 859 

[silvin-lubecki]

LGTM, thanks @stoffus 👍 I works well on my zsh 🚀

[thaJeztah]

LGTM