Spack dockerfile generation failed on main, but not on PR branch

[glemieux]

For some reason, while the PR branch action passed the "Generate dockerfile with spack" step (https://github.com/NGEET/containers/actions/runs/16431004329/job/46432237331), once merged with main this step is now failing: https://github.com/NGEET/containers/actions/runs/16434589443/job/46442162052

[glemieux]

This is actually the consequence of merging in both #4 and #5 which were not tested together. Both were based off of the same commit on main and tested in parallel.