Installed Docker on Mac but can't find the command

I'm unsure which instructions you followed to install via pip, however docker-3.4.1 is a very old version, so you have worse issues at foot than your PATH not containing the docker binary.

I recommend undoing your installation via pip and installing Docker for Mac the correct way. I believe the current stable version for MacOS is 18.03.

https://store.docker.com/editions/community/docker-ce-desktop-mac

First go and uninstall whatever you did with pip uninstall docker

Then go here https://docs.docker.com/docker-for-mac/install/ and dowload docker and follow the instructions and install it.