Fix CUDA Docker image bug + Improve Dockerfile + Add requirements-extra.txt

[FrancescoCasalegno]

Fixes NVIDIA/nvidia-docker#63

Description

1. Fix the public key is not available: NO_PUBKEY bug by using the workaround suggested in GPG error "public key is not available" in Ubuntu 20.04 CUDA 11.4.0 image while building NVIDIA/nvidia-container-toolkit#257
2. Remove PIP_INDEX_URL pointing at BBP internal Python index from the Dockerfile (otherwise pip install neurots will look for the wrong package in our index instead of PyPI!)
3. Introduce requirements-extra.txt to cover the requirements of extras_require.

How to test?

1. Running docker build should now succeed.
2. Running pip install .[dev,test] inside the container should install morphoclass and confirm that all dependencies are already there.

Checklist

• This PR refers to an issue from the issue tracker.
  (if it is not the case, please create an issue first).
• setup.cfg, requirements.txt, and constraints.txt updated with new dependencies.
  (if needed)
• All CI tests pass.