🚧 feat: autocompletion installer 🚧

[Benehiko]

- What I did
Override the cobra completion command to add extra functionality such as automatically installing completions on the user machine. The behavior of the completion command should still generate cobra commands.

- How I did it
Added a completion cobra.Command onto the root command.

- How to verify it
Run docker completion --help

Some examples:

docker completion zsh --manual
mkdir -p .docker/completions && docker completion zsh > .docker/completions/_docker
cat <<EOT >> .zshrc
# The following lines have been added by Docker to enable Docker CLI completions.
fpath=(.docker/completions/_docker $fpath)
autoload -Uz compinit
compinit
EOT
# End of Docker Completions⏎ 

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