formalize spack/venv recipes into standalone files

instead of adding them inside Dockerfile, they could be their own independent files

cc @bartgol

Start by reviewing the Dockerfile and locating the inline spack and venv recipes mentioned in the issue. Determine how those recipes are currently added and which standalone files the container build should use. The work is done when the recipes are independent files rather than embedded in the Dockerfile, with the existing builds still working.

Written by the indexing model from the issue text.