Making bl-themes and bl-plugins persistent

[kylethedeveloper]

There is no persistent way to add new themes or plugins in Docker installation.

Mounting a volume like bl-contents would not work as mount operation removes the existing files in the container. The only way for now is to manually download bl-themes and bl-plugins contents and put it into host directory, then run the container with mounting.

I suggest an enhancement about this issue. I already started working on this, may send a pull request soon.

Thanks in advance.