More fine-grained customization of the OCI image reference

[Djaytan]

Actually, the container.image property is a string one: link.

The goal here is to convert it to object and expose three sub-properties:

• registry: hostname server from where the OCI image must be pulled (default value: docker.io)
• name: the image name (default value: djaytan/papermc-server)
• tag: the image tag (default value: latest)
• digest: the checksum for integrity check (default value: none)

This way, as a user, I can just override the tag without bothering about anything else, thus keeping an override config concise and simple:

container:
  image:
    tag: '1.21.4-v2.11'
    digest: 'sha256:7136fadbae3f4acf8c4f470d047516af0fda4f3298de85aa14f7b9f8347019fe'

=> Principal: convention over configuration.

Alternatively, or complementary, I can as well create my own custom OCI image based on the Djaytan's one while still using this Helm chart:

container:
  image:
    registry: ghcr.io
    name: my-org/my-repo
    tag: latest

Or I may need to override specifically the registry in order to rely on a mirror to avoid facing Docker Hub rate limits:

container.image.registry: my-mirror.io