[FEATURE] Use `sociIndex` as variable name consistently

[Kern--]

Description

Many places in the code base use index as a shorthand for sociIndex. With v2, we also have OCI indexes (also called image indexes) which makes the usage of index ambiguous. We should rename the variables to make this more clear.

Describe the solution you'd like

We should use the following:
sociIndex -> the SOCI index that's used to lazily load an image for a specific platform
imageIndex -> the wrapper image that acts as a soci-enabled image in v2.

Describe any alternative solutions/features you've considered

No response

Any additional context or information about the feature request

No response