Mapping between docker tags and appc manifest labels

[sgotti]

I think that a docker image tags should be used to detect the version label in the manifest. Now latest is used by I have some doubts about the latest pattern in appc/spec (see appc/spec#60 and appc/spec#73) and the usage of latest as a valid version.

Some questions and notes.

• Docker images can have multiple tags. Which one to use for the version label?
• Given the previous point, what to do with the remaining tags?
• As docker tags can come and go from an image with the same Id (for example the latest tag can be moved to another image), while aci manifest labels are tied to the ACI imageID (as changing a label, so the manifest will change its sha512), if the tags converted to manifest labels (previous points) changes in the docker registry a new aci with different imageID will be created. I don't see this as a problem but as a different behavior. The cas, should handle this. (for example the implementation done in cas: Introduce latest pattern handling for image fetching. rkt/rkt#394 handles the image import time and the latest pattern).

[jonboulle]

The relationship between Docker tags and AC labels is a bit awkward because, as you've highlighted, the former are mutable and tied to repositories whereas labels in an image manifest are immutable after the image is created. So I am not really sure we should do anything with tags in docker2aci since it is at best misleading and at worst going to inaccurate almost immediately after the image is created. Instead this should be handled by the repository (which, locally, is the CAS).

Also filed #15 to clarify the version label.

[sgotti]

@jonboulle thanks. I totally agree with you.