Add docker distribution manifest list v2 type to accept headers

What does this MR do and why?

This MR adds application/vnd.docker.distribution.manifest.list.v2+json to the Accept header that's sent when getting the manifest off Docker hub. That'll add the support for arm images for the dependency proxy.

• The feature flag rollout issue: [Feature flag] Rollout of `add_docker_distribut... (#419797 - closed)
• The documentation changes: Dependency proxy docs: Update "exec format erro... (!127636 - merged)

Screenshots or screen recordings

With master branch

With the MR's branch

How to set up and validate locally

1. Setup dependency_proxy how to guide

2. In rails console create a new group and enable the feature flag:

   g = FactoryBot.create(:group, name: 'my_group')
   Feature.enable(:add_docker_distribution_manifest_list_v2_type_to_accept_header, g)

3. Pull the image through the dependency proxy

   docker run gdk.test:3000/my_group/dependency_proxy/containers/node:14

   The image should be pulled correctly without a warning like:

   WARNING: The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested

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.

Related to #325669 (closed)