Clone fixed version for the facebook/demucs to prevent issues with new versions

[xserrat]

Hello there! 👋🏽

I was thinking about the recent issue regarding the incompatibility of the existing docker implementation with the latest version of facebook/demucs.

Checking the repository @adefossez , I've seen that you have a branch per version so we could set this branch when cloning the repo in the Dockerfile:

RUN git clone -b <version> --single-branch https://github.com/facebookresearch/demucs /lib/demucs

Then, if a new version is released, we can check the new version without affecting the current usage with the old version.

What do you think?

Any suggestions are welcomed 😄