Add mirror config for our gcp project

A zot registry has been deployed in sylva's GCP project to prevent having our test clusters being rate-limited.

It's configured to serve as a mirror for most registries we're using in the CI.

• Should we only configure it for dockerhub (since the rate-limiting only came from it so far)?
• Should we also configure it for ghcr ? https://status.linuxserver.io/endpoints/docker-registries_ghcr-io
• Should we also configure it for quay ? https://status.linuxserver.io/endpoints/docker-registries_quay-io

Test MR: sylva-projects/sylva-core!8136 (merged)