docker-magento: Copying /var/www/html from appdata volume fails

I have followed the setup guide using the composer auth method and everything installs and runs great. My problem is that when I attempt to copy the filesystem as described in the README, I get the following:

C:\Users\NathanS\mage2>docker cp 81a:/var/www/html ./
symlink \var\www\.composer C:\Users\NathanS\mage2\html\var\composer_home: A required privilege is not held by the client.

I have tried using the command prompt as well as git-bash (Mingw64) both with the same result. I have tried many different things to bypass this issue but have had no success. I also can’t find anybody else talking about this problem.

I am on 64bit Windows 10 Enterprise running as an Administrative user.

What can I do to get this to work?