Copied from the official Docker-for-mac documentation (thanks Brett for the updated doc pointer):
Docker Desktop for Mac comes with scripts to enable completion for the docker, docker-machine, and docker-compose commands. The completion scripts may be found inside Docker.app, in the Contents/Resources/etc/ directory and can be installed both in Bash and Zsh.
Bash has built-in support for completion To activate completion for Docker commands, these files need to be copied or symlinked to your bash_completion.d/ directory. For example, if you installed bash via Homebrew: