Apptainer CI

[jlenain]

This PR introduces a new CI workflow where:

• nectarchain is shipped as a conda environment, and buit as an Apptainer/Singularity container on selected events (releases, pull requests, merges into master),
• the container is deployed on the GitHub Container Registry, i.e. visible as a package of the cta-observatory organization, on non-pull-request events, such as releases.

Users can then use such containers with:

apptainer shell oras://ghcr.io/cta-observatory/nectarchain:latest

TL;DR

We will now have fully automatic Apptainer containers built and deployed on releases!

@kosack , @maxnoe , would that be deemed acceptable ?

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (16418f7) 0.53% compared to head (9a2af1f) 0.53%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master     #53   +/-   ##
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  Coverage    0.53%   0.53%           
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  Files          23      23           
  Lines        2434    2434           
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  Hits           13      13           
  Misses       2421    2421           

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