Fixes

docker/lib/dependabot/docker/update_checker.rb

@@ -125,10 +125,10 @@ def version_tag_up_to_date?(version)
 
       def digest_up_to_date?
         dependency.requirements.all? do |req|
-          next true unless req.fetch(:source)[:digest]
-          next true unless (new_digest = latest_digest)
+          next true unless (digest = req.fetch(:source)[:digest])
+          next true unless (new_digest = updated_digest)
 
-          req.fetch(:source).fetch(:digest) == new_digest
+          digest == new_digest
         end
       end
 
@@ -232,7 +232,11 @@ def canonical_version?(tag)
       end
 
       def updated_digest
-        @updated_digest ||= digest_of(latest_version)
+        @updated_digest ||= if latest_version.match?(DIGEST)
+                              latest_digest
+                            else
+                              digest_of(latest_version)
+                            end
       end
 
       def tags_from_registry

docker/spec/dependabot/docker/update_checker_spec.rb

@@ -143,7 +143,7 @@
       let(:source) { { digest: digest } }
 
       let(:headers_response) do
-        fixture("docker", "registry_manifest_headers", "ubuntu_17.10.json")
+        fixture("docker", "registry_manifest_headers", "generic.json")
       end
 
       before do