Virtualbox doesn't mount local shared folder, if there is a windows mapped drive before the locale one.

The docker-toolbox uses VirtualBox  to boot a small Linux (boot2docker.iso) to provide the
docker engine.

To use local directories inside the containers, the linux-system in VirtualBox must mount the Windows directories.

There is an option named '--virtualbox-share-folder' to be used with the docker-machine.exec create
or you can map the Shared-Folders in the VirtualBox GUI:

The 'Name' part is the mount-Point in the linux-system.

For the containers you can use the "-v" volume option to mount the directories into the container.

For me, the L: drive was not available when I started the virtual machine with

docker-machine.exe start <virtualLinuxSystem> 

and as a result of this, also the local directory after the mapped networkdrive was not available in the
virtual machine and in the container.

After changing to the mapped networkdrive within the Windows Explorer, the mounts are available in the virtual linux system: