[ECR] [Add Docker-Content-Digest to v2 api in response headers]: Implement part of OCI spec

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**Tell us about your request**
When using the public ECR v2 API, calling /v2//manifests/ resource it should return the Docker-Content-Digest of the uploaded blob. As specified in the [open container specification](https://github.com/opencontainers/distribution-spec/blob/main/spec.md#appendix).

> A GET request to an existing manifest URL MUST provide the expected manifest, with a response code that MUST be 200 OK. A successful response SHOULD contain the digest of the uploaded blob in the header Docker-Content-Digest.

> The Docker-Content-Digest header, if present on the response, returns the canonical digest of the uploaded blob which MAY differ from the provided digest. If the digest does differ, it MAY be the case that the hashing algorithms used do not match. See [Content Digests](https://github.com/opencontainers/image-spec/blob/v1.0.1/descriptor.md#digests) [apdx-3](https://github.com/opencontainers/distribution-spec/blob/main/spec.md#appendix) for information on how to detect the hashing algorithm in use. Most clients MAY ignore the value, but if it is used, the client MUST verify the value against the uploaded blob data.

**Which service(s) is this request for?**
ECR

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I'm currently using the manifest digest for comparing purposes. Without the digest of the manifest present in the headers I can't compare.

**Are you currently working around this issue?**
Not, I don't support public ecr currently.

Start with the public ECR v2 manifest endpoint described in the issue and compare its behavior with the linked OCI distribution specification. Identify the service implementation or tracking entry for GET /v2//manifests/; done means successful responses include the canonical Docker-Content-Digest header for the uploaded manifest.

Written by the indexing model from the issue text.