Add support for OCI image indexes in Dependency Proxy

What does this MR do and why?

This is a fix for Dependency proxy fails to pull OCI image indexes (#385217 - closed). Please read the issue description to understand the problem, the solution, and how it was implemented.

Screenshots or screen recordings

Screenshots included in the How to set up and validate locally section.

How to set up and validate locally

Assumptions

• GDK set up with an Ultimate license and configured as described here;
• Docker-compatible client installed (Docker CLI, colima, etc.);
• GitLab Rails listening on gdk.test:3000 (can be other, just replace it on the instructions as needed);
• gitlab-org group with the Dependency Proxy enabled.

Before

Running the master version of this repository (before this MR is merged), confirm that:

• It IS possible to pull non-OCI index images:

  ❯ docker pull gdk.test:3000/gitlab-org/dependency_proxy/containers/alpine
  Using default tag: latest
  latest: Pulling from gitlab-org/dependency_proxy/containers/alpine
  Digest: sha256:e2e16842c9b54d985bf1ef9242a313f36b856181f188de21313820e177002501
  Status: Image is up to date for gdk.test:3000/gitlab-org/dependency_proxy/containers/alpine:latest
  gdk.test:3000/gitlab-org/dependency_proxy/containers/alpine:latest

• It IS NOT possible to pull OCI index images:

  ❯ docker pull gdk.test:3000/gitlab-org/dependency_proxy/containers/grafana/mimir
  Using default tag: latest
  Error response from daemon: manifest for gdk.test:3000/gitlab-org/dependency_proxy/containers/grafana/mimir:latest not found: manifest unknown: OCI index found, but accept header does not support OCI indexes

• Only the non-OCI index images show up in the UI:

  

After

Running this branch's version (master-patch-5435), confirm that:

• It IS possible to pull non-OCI index images already cached (sanity check):

  ❯ docker pull gdk.test:3000/gitlab-org/dependency_proxy/containers/alpine
  Using default tag: latest
  latest: Pulling from gitlab-org/dependency_proxy/containers/alpine
  Digest: sha256:e2e16842c9b54d985bf1ef9242a313f36b856181f188de21313820e177002501
  Status: Image is up to date for gdk.test:3000/gitlab-org/dependency_proxy/containers/alpine:latest
  gdk.test:3000/gitlab-org/dependency_proxy/containers/alpine:latest

• It IS possible to pull non-OCI index images not yet cached (sanity check):

  ❯ docker pull gdk.test:3000/gitlab-org/dependency_proxy/containers/busybox
  Using default tag: latest
  latest: Pulling from gitlab-org/dependency_proxy/containers/busybox
  1487bff95222: Pull complete
  Digest: sha256:2c8ed5408179ff4f53242a4bdd2706110ce000be239fe37a61be9c52f704c437
  Status: Downloaded newer image for gdk.test:3000/gitlab-org/dependency_proxy/containers/busybox:latest
  gdk.test:3000/gitlab-org/dependency_proxy/containers/busybox:latest

• It IS possible to pull OCI index images not yet cached (fix):

  ❯ docker pull gdk.test:3000/gitlab-org/dependency_proxy/containers/grafana/mimir
  Using default tag: latest
  latest: Pulling from gitlab-org/dependency_proxy/containers/grafana/mimir
  Digest: sha256:63d891f40d575825d4d22a595a5e71c2bb7b4786bb8539e3d893e0fe0f3139bf
  Status: Image is up to date for gdk.test:3000/gitlab-org/dependency_proxy/containers/grafana/mimir:latest
  gdk.test:3000/gitlab-org/dependency_proxy/containers/grafana/mimir:latest

• It IS possible to pull OCI index images already cached (sanity check):

  ❯ docker pull gdk.test:3000/gitlab-org/dependency_proxy/containers/grafana/mimir
  Using default tag: latest
  latest: Pulling from gitlab-org/dependency_proxy/containers/grafana/mimir
  Digest: sha256:63d891f40d575825d4d22a595a5e71c2bb7b4786bb8539e3d893e0fe0f3139bf
  Status: Image is up to date for gdk.test:3000/gitlab-org/dependency_proxy/containers/grafana/mimir:latest
  gdk.test:3000/gitlab-org/dependency_proxy/containers/grafana/mimir:latest

• All cached images show up in the UI:

  

  We found a non-blocking frontend issue now: Adds digest label & copy button for dependency ... (#396383 - closed).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.