If you have already homebrew bash-completion installed just install the docker completion script into the bash_completion.d
curl -XGET https://raw.githubusercontent.com/docker/cli/master/contrib/completion/bash/docker > $(brew --prefix)/etc/bash_completion.d/docker
Note: If you do not have homebrew bash-completion installed, follow these instructions to install it before you execute the line above.
Note: the completion depends on some functions defined in debian bash-completion. Therefore, just sourcing the docker completion script as described in completion/bash/docker may not work. If you try to complete docker run (by hitting TAB) you may get an error like __ltrim_colon_completions: command not found. This could mean that you have not installed the bash-completion scripts.