This allows to build the container that will call the generate-frozen-requirements
script.

That will be usable from ci or other sysadm.

Use:
```
cd swh-apps/scripts
docker build -t build-deps .
cd ../../swh-apps
docker run -v $PWD:/src --user 1000 build-deps $APP
```

This allows better caching of modules with no pyproject.toml