Following the steps in Windows Instructions (Docker Toolbox), I try to open the Samba workdir at \\192.168.99.100\workdir, which is the same ip address shown by docker-machine ip.
I get a Windows Security popup that says "Enter network credentials Enter your credentials to connect to: 192.168.99.100" It will not allow entering nothing, and all my guesses do not grant access (admin, user, smbuser / password). Following the instructions on this page, I have not set up a username or password for the docker / virtualbox environment.