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.

First go and uninstall whatever you did with pip uninstall docker

Then go here and dowload docker and follow the instructions and install it.