It would be great to have an option to run in a "read-only" mode, where the cache plugin reads from S3, and stores the cache entries locally on disk, but does not attempt to write any data back to S3.

This feature might be useful in untrusted environments to prevent cache poisoning attacks.

I would suggest to add a new boolean flag --read-only, which defaults to false (keeping the current behavior). Similarly to MinUploadSize this would just skip the upload step.

If you agree with this proposal, I can also contribute this feature