Add new mount point for docker

[Les-Wet]

Spent a long while debugging this so making a PR to help others avoid the same. When using the pre-built Docker image, another mount point has to be added to allow the container to see the changes to the unbricked assembly code. The current command in the README will result in the following error (or similar):

[ERROR] (mdbook::preprocess::links): Error updating "{{#include ../../unbricked/vblank-interrupts/main.asm:vblank-interrupt}}", Could not read file for link {{#include ../../unbricked/vblank-interrupts/main.asm:vblank-interrupt}} (/code/src/part2/../../unbricked/vblank-interrupts/main.asm)

And the tutorial will not display the correct code.

This PR adds a mount point to resolve this issue:

--mount "type=bind,source=$(pwd)/unbricked,target=/code/unbricked" 

Building the container locally avoids this problem since it copies the code into the container directly:

COPY . /code

[Les-Wet]

Sorry I should specify, this isn't actually changing the dockerfile or image in any way. This is just changing the instructions on how to use it in the README.

[avivace]

Good catch, thanks @Les-Wet