Containerized environment for development/experimentation

[A669015]

We could integrate in the repository a Dockerfile to build an OCI image embedding all the py4cast requirements.
The image could be built by the CI and published (on github.io or docker hub) as the base development/experimentation environment.

The README could document the usage of this image to train models with py4cast, particularly in the case of an HPC platform using Singularity.

It would allow to simplify and rationalize the environment reference files (today 3 different files are provided requirements.txt, env_conda.yaml, env.yaml), keeping only the requirements.txt and documenting the usage of the OCI environment instead of conda.