fix: Manifest Tests Failing on Redis Image

[ollypom]

Related Issue

TestHeadManifest and TestGetManifest were failing as the upstream public.ecr.aws/docker/library/redis:7 have switched from Docker Manifest Lists to OCI Image Indexes.

$ crane manifest public.ecr.aws/docker/library/redis:7 | jq -r '.mediaType'
application/vnd.oci.image.index.v1+json

See failing tests:

=== RUN   TestGetManifest
{"level":"info","RequestId":"abcd-1234-test-get-manifest","time":"2024-02-12T12:00:44Z","message":"Initializing registry client"}
    registry_test.go:76: Incorrect manifest media type. Expected application/vnd.docker.distribution.manifest.list.v2+json but got application/vnd.oci.image.index.v1+json
--- FAIL: TestGetManifest (0.37s)

Proposed Changes

Added a new constant for the Image Index and updated the tests. I have validated locally with make test.

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