Improve image references in Helm values by adding support for digests and registry fields

[outofrange]

I want to suggest the following improvements for setting postfix / postfix-exporter image references through mail Helm values:

• add a digest key to image dicts - setting one is currently possible by using the tag field, but it's not quite obvious, and (probably) only works with tag@digest formats, not with digest-only (see also comment in #263)
• add global.imageRegistry key, defaulting to docker.io, which will be used as a prefix to repositorys (joined by / if not already part of global.imageRegistry value)
• add a registry key (default "") to image dicts, allowing to override default registry from globals for each image

This follows a common structure recognized by at least Renovate - although I have to check if this is also true for global.imageRegistry.

[outofrange]

It might be worth splitting digest and registry fields into two issues, since registry could be a breaking change when not done carefully - people with older values, having image.repository set to registry.example.com/postfix, would end up with references like docker.io/registry.example.com/postfix, until they move their registry.example.com definition to image.registry.

Could be handled by Helm helper logic, only prefixing with default docker.io registry when first part of repository isn't a domain / doesn't contain . - need to look up how other charts handled adding image.registry to an already released chart.