Define a default value for the `container.image` property

[Djaytan]

Actually, the container.image property is mandatory: source.

In order to allow quick starts for new users (typical by experimenting with the Helm chart), we aim to reduce as much as possible the number of mandatory properties.

We can achieve that by assigning a default to this property.

The first thing to note: the JSON Schema is not aligned with the reality since in practice a default value is already defined for the property.....

TO COMPLETE