If you try to start a docker machine using docker-machine start <machinename>, you might receive this error message:

Progress state: NS_ERROR_FAILURE
VBoxManage: error: Failed to create the host-only adapter
VBoxManage: error: VBoxNetAdpCtl: Error while adding new interface: failed to open /dev/vboxnetctl: No such file or directory
VBoxManage: error: Details: code NS_ERROR_FAILURE (0x80004005), component HostNetworkInterface, interface IHostNetworkInterface
VBoxManage: error: Context: "int handleCreate(HandlerArg*, int, int*)" at line 66 of file VBoxManageHostonly.cpp

To resolve this issue, you have to setup the vboxdrv kernel module using

sudo /etc/init.d/vboxdrv setup