Mark outdated images

[eine]

The tags of some of the images in the "Images" tree view are likely to get outdated when maintainers push new versions to the corresponding registries. Similarly to how modifications are marked in git repos, I think it would be useful to have a symbol added to outdated images.

Related to #1313, this feature would help users decide when they need to explicitly pull the image.

[bwateratmsft]

I've been investigating this some. I can't find any documentation about the Docker Hub API (the old one, not the Registry V2 API). The Registry V2 API is a non-starter for this purpose because it requires authentication, even to access public images.

There is a JSON endpoint, e.g. https://hub.docker.com/v2/repositories/library/nginx/tags/latest, but the last_updated time in that result is unreliably far away from the actual manifest time of the image (e.g. for this nginx:latest, 54 minutes; for alpine:latest, 2 hours 22 minutes). IMO this is too far and too variable to be dependable.

[bwateratmsft]

Interestingly I found that you do not need to provide anything in the Authorization header to get a valid token from the auth.docker.io/token endpoint; at least for public scopes like repository:library/nginx:pull. With that token you can get a proper manifest from https://registry-1.docker.io/v2/library/nginx/manifests/latest to do an exact check.