handler for error response from docker image tag fetch (#11594)

docker/lib/dependabot/docker/update_checker.rb

@@ -261,6 +261,12 @@ def tags_from_registry
       rescue RestClient::ServerBrokeConnection,
              RestClient::TooManyRequests
         raise PrivateSourceBadResponse, registry_hostname
+      rescue JSON::ParserError => e
+        if e.message.include?("unexpected token")
+          raise DependencyFileNotResolvable, "Error while accessing docker image at #{registry_hostname}"
+        end
+
+        raise
       end
 
       sig { returns(T.nilable(String)) }

docker/spec/dependabot/docker/update_checker_spec.rb

@@ -582,6 +582,18 @@ def stub_tag_with_no_digest(tag)
         end
       end
 
+      context "when there is ParserError response from while accessing docker image tags" do
+        before do
+          stub_request(:get, repo_url + "tags/list")
+            .to_raise(JSON::ParserError.new("unexpected token"))
+        end
+
+        it "raises" do
+          expect { checker.latest_version }
+            .to raise_error(Dependabot::DependencyFileNotResolvable)
+        end
+      end
+
       context "when TooManyRequests request error" do
         before do
           stub_request(:get, repo_url + "tags/list")