Singularity / docker image features

[agrouaze]

Change Summary

This PR brings modifications to allow a user to use a singularity image or a docker image instead of conda environement.

Related issue number

addressed issues : #204

Checklist

• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable

[agrouaze]

@keewis First of all, do you think this PR worth to be done? (perhaps other solutions allow to use singularity image with jupyter-fwd ?)
Do you have any experience of CI unit tests using singularity images?
I guess that we don't need to ship any .sif file within the repository but so far I only found https://github.com/singularityhub/github-ci and it contains instructions to test a singularity image that would be the output of a release while I would rather need example on how to test a library for which the singularity image is an input element.
I guess jupyter-forward lib does not aim at providing singularity images but it aims at making possible users execute their notebooks with their own images.

edit: I saw that singularity will be replaced by apptainer, for now I don't know what could be implications on the code proposed.