use dohead method to fetch blob metadata for Docker tags (#12712)

* use `dohead` method to fetch blob metadata for Docker tags

* change stub from get to head

Author: robaiken

docker/lib/dependabot/docker/update_checker.rb

@@ -224,7 +224,7 @@ def get_tag_publication_details(tag)
 
         blob_info = with_retries(max_attempts: 3, errors: transient_docker_errors) do
           client = docker_registry_client
-          client.blob(docker_repo_name, first_digest)
+          client.dohead "v2/#{docker_repo_name}/blobs/#{first_digest}"
         end
 
         last_modified = blob_info.headers[:last_modified]

docker/spec/dependabot/docker/update_checker_spec.rb

@@ -1489,9 +1489,8 @@ def stub_tag_with_no_digest(tag)
         blob_headers =
           fixture("docker", "image_blobs_headers", "ubuntu_17.10_38d6c1.json")
 
-        stub_request(:get, repo_url + "blobs/sha256:9c4bf7dbb981591d4a1169138471afe4bf5ff5418841d00e30a7ba372e38d6c1")
-          .and_return(status: 200, body: fixture("docker", "image_blobs", "ubuntu_17.10_38d6c1.json"),
-                      headers: JSON.parse(blob_headers))
+        stub_request(:head, repo_url + "blobs/sha256:9c4bf7dbb981591d4a1169138471afe4bf5ff5418841d00e30a7ba372e38d6c1")
+          .and_return(status: 200, headers: JSON.parse(blob_headers))
       end
 
       it { is_expected.to eq("17.10") }