Accept compose file as a workflow input instead of via GitHub's cache

[axisofentropy]

We'd like to support compose files with secrets. While GitHub's cache access model is decent, it's not intended to keep secrets.

Compose files are often larger than most GHA workflow inputs and may have special characters, so we should first see if they'll need special handling such as base64 encoding.

[axisofentropy]

This may justify a v3 version bump for the reusable workflow.

Don't forget documentation! Examples will be critical to prepare the input value within end users' workflows.