Replace EnvironmentDefined's data.image with new link type

[magnusbaeck]

Description

The data.image member of EiffelEnvironmentDefinedEvent is defined like this:

A string identifying e.g. a Docker image. While not a perfect description of an environment, in many cases it is both sufficient and conducive.

Since a Docker image (and other kinds of execution environment images like VMDK or AMI) can be described with artifacts it's not clear why this field was introduced instead of having a link to an ArtC describing the image. Maybe made more sense when artifacts were identified with GAVs?

I suggest this field should be deprecated and replaced with a new link type, perhaps IMAGE, with EiffelArtifactCreatedEvent as the sole possible target.

Motivation

This would allow much more useful Eiffel environment definitions since the execution environment could be specified with links between events instead of a somewhat ambiguous data field that requires the consumer to make additional queries to other systems in order to get back into the Eiffel domain.

Exemplification

Here's what a DAG could look like, allowing a graph traversal to list which OS packages that were installed in the execution environment where the artifact was produced:

Benefits

This would improve execution environment traceability within the Eiffel DAG.

Possible Drawbacks

None I can think of, except maybe that removing the data.image would be a backward incompatible change that could be disruptive.