How-to activate completion for docker client in zsh and prezto

• get zsh completion

curl -fLo ~/.zprezto/modules/completion/external/src/_docker \
https://raw.githubusercontent.com/docker/cli/master/contrib/completion/zsh/_docker

• activate completion plugin in .zpeztorc

zstyle ':prezto:load' pmodule \
...
  'completion' \
...