Add tmpfs to container builder

[manuelamateo]

I would like to use the --tmpfs command when building a container to transform volumes form a given image to in-memory file systems for testing (https://docs.docker.com/storage/tmpfs/).
As far as I saw, this isn't supported at the moment. Is there another way to accomplish my goal? Otherwise I could contribute this functionality.

[mariotoffia]

Thanks for your PR!! :) It is merged