Support Docker registries that don't issue Docker-Content-Digest headers

[nbyl]

What would you like Renovate to be able to do?
The docker digest feature is really helpful to update base images, when the contents of a docker tag changes. The implementation of the retrieval of the current digest is based in the Docker-Content-Digest header. According to the offical documentation this header may be added. One example which does not add this header is RedHats registry at registry.access.redhat.com. For these registries renovate should have a fallback.

Describe the solution you'd like
If would like to use the blueprint of tools like the mechanism of skopeo to build a more resilient way of retrieving this digest.

Describe alternatives you've considered
An alternative would be to download the image and retrieve it from the local docker daemon. This is a pretty big overhead.

I will be working on this feature at my fork to see if this a possible way to go.

[rarkins]

For my hosted app, I don’t want to download entire images just to get the digest. For self hosted I suppose we could enable this behind an admin feature flag, but ideally there’s a better way!

[renovate-bot]

🎉 This issue has been resolved in version 13.39.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀