[docker/cli] contrib/completion/zsh doesn't complete compose

### ROOT CAUSE The `contrib/completion/zsh/_docker` file does not handle the `compose` command properly. The completion function lacks a case statement for `compose`, preventing subcommand completion. ### CODE FIX Update the Zsh completion function to include the `compose` command and its subcommands.

zsh
# In contrib/completion/zsh/_docker, add:
  compose) {
    _docker_compose_COMPLETE=($(_docker_compose "$1"))
    return 0
  }


This adds a case for `compose`, enabling subcommand completion when `docker compose` is used.