Documentation: File ownership and permissions

Problem

Neither the general workflow documentation nor this repo's README mention file ownership or permissions at all, but these things can cause this action to fail, see #133.

Suggestion

The documentation should specify the assumptions / prerequisites / requirements to successfully cache a file. Does it need to be owned by a particular user? Which user? Does it need to be readable by everyone? Is there a way to make this action work with all files regardless of their ownership and permissions?

This would help to ensure that users who follow the action's instructions end up wtih a working workflow instead of a broken one.

Note that the documentation for Docker actions says this:

... so some actions will run as root. The documentation should discuss best practices for how to ensure that root-owned files created by such actions can be cached.

Start with the repository README and the general workflow documentation linked in the issue, then review #133 for the reported failure. Determine the cache action's file ownership and permission requirements, including root-owned files created by Docker actions. Done means the documentation clearly states the prerequisites and best practices for caching such files.