ContainerRegistry: Tolerate a missing Location header in the manifest upload response (#27)

### Motivation

According to distribution spec, the manifest `PUT` response [MUST contain a Location header](https://github.com/opencontainers/distribution-spec/blob/main/spec.md#pushing-manifests) providing a URL from which the saved manifest can be downloaded. The same requirement applies to blob `PUT`s.   However some registries return URLs which cannot be fetched, and ECR does not set this header at all. 

The location header is not currently of critical important.   If the header is not present, we can create a suitable value based on the spec for [manifest downloads](https://github.com/opencontainers/distribution-spec/blob/main/spec.md#pulling-manifests).

### Modifications

Generate a suitable manifest location URL when the registry does not provide one.

### Result

Uploads to ECR will no longer fail because of the missing `Location` header in the manifest `PUT` response.

### Test Plan

Automated tests continue to pass; manually tested with ECR.

Author: euanh

Sources/ContainerRegistry/Manifests.swift

@@ -29,10 +29,14 @@ public extension RegistryClient {
             decodingErrors: [.notFound]
         )
 
-        guard let location = httpResponse.response.headerFields[.location] else {
-            throw HTTPClientError.missingResponseHeader("Location")
-        }
-        return location
+        // The distribution spec says the response MUST contain a Location header
+        // providing a URL from which the saved manifest can be downloaded.
+        // However some registries return URLs which cannot be fetched, and
+        // ECR does not set this header at all.
+        // If the header is not present, create a suitable value.
+        // https://github.com/opencontainers/distribution-spec/blob/main/spec.md#pulling-manifests
+        return try httpResponse.response.headerFields[.location]
+            ?? registryURLForPath("/v2/\(repository)/manifests/\(manifest.digest)").absoluteString
     }
 
     func getManifest(repository: String, reference: String) async throws -> ImageManifest {