I have been heavily using Docker recently to help me develop under different environment. And i love it much, and you should also. I use Docker Toolbox (DTB) because i still want to use VirtualBox for other projects. Docker Toolbox uses VirtualBox headless, so it is going well running with VirtualBox GUI.
The problem is the default images created by Docker is under drive C:. Located at C:\Users\[username]\.docker\machine\machines\default and usually this drive do not have much space left. So i need to move the created VM image to other location.
The idea is moving disk.vmdk to other location. Vmdk is VirtualBox virtual machine created by docker. So, you need to change VirtualBox configuration to allow access vmdk in other location.
I've been searching all over the net and here is the simple way and work for my use case.
Stop Docker Machine
Move disk.vmdk to other location
Copy disk.vmdk from C:\Users\[username]\.docker\machine\machines\default to other location, i.e D:\Docker\machine\default.
Edit default.vbox and edit the new disk.vmdk location
Go to VirtualBox configuration file default.vbox located at C:\Users\[username]\.docker\machine\machines\default\default and edit the line that specify disk.vmdk to match your new location (i.e. to D:\Docker\machine\default).
Start Docker Quickstart Terminal
Check if the disk already used what we defined above