Docker set up to support reading checkpoint_file from config.json and documentation

[ntlex]

Hi,

Created a PR to support passing the model's checkpoint_files in a docker container set up. Here's a sort description of the changes:

1. Even though in label_studio_ml/default_configs/_wsgi.py.tmpl there is support for reading the checkpoint_file through config.json (see method get_kwargs_from_config()) the json import is missing as well as making the kwargs available to the uWSGI app.
2. Added config.json in the default_configs
3. Updated Dockerfile to copy the config.json as well as the checkpoint_file that should be placed in ./models/
4. Updated README.md with the instructions on using the backend with docker, including considering the cloud storage credentials and checkpoint file exposure