As per spec, "Location" header is typically of the form:
Location: /v2//blobs/uploads/<session_id>

Also,

Though the URI format (/v2/<name>/blobs/uploads/<session_id>) for the Location header is specified, clients SHOULD treat it as an opaque url and SHOULD never try to assemble it.

However, docker distribution appears to return a fully qualified "Location" header value of the form:
https://<server_ip>:<server_port>/v2//blobs/uploads/<session_id>

This behavior needs further clarification. What should be a compliant client's expectation from the server? In one case, it needs to prepend the Location header with the (scheme,address,port) and in the other, not.