This is an issue with PhpStorm 2016.3.3 on Windows. I encountered the following error trying to configure a Docker container:
I can make the immediate error-message go away by going into the Build, Execution, Deployment > Docker
and add the same folder to the "VirtualBox shared folders" selection...
... and that unblocks the file-picker ...
...But unfortunately this still does not mount when I run the container.