Add an option to mount directory inside the docker container.

[MrCyjaneK]

As we were talking in the discord, it would be cool to be able to mount directory inside of the docker image.
For security reasons, this should be done only in the ui, or in .abstruse.yml only relative to a path set in variable and in .abstruse.yml

Use case is when you have to deal with a large amount of data, and cache is not enough/or you have to do incremental builds.

Downside is that you need to have the path being accessible for all the workers (or assign a specific worker #507). For me sshfs will do the trick.

The option should allow to pick a specific path on host, inside the docker image, and allow to pick if you want it to be writable or read only.

TODO:

• ability to mount directories inside of docker container
• specifying mount type (for example: read only)