Docker: support mixed case version suffixes (RC)

Downcase the docker version tag before finding the latest tag which is
compatible with the inherited Bundler version class.

Fixes: dependabot/feedback#779

docker/lib/dependabot/docker/update_checker.rb

@@ -283,7 +283,7 @@ def prerelease?(tag)
       def numeric_version_from(tag)
         return unless tag.match?(NAME_WITH_VERSION)
 
-        tag.match(NAME_WITH_VERSION).named_captures.fetch("version")
+        tag.match(NAME_WITH_VERSION).named_captures.fetch("version").downcase
       end
 
       def registry_hostname

docker/spec/dependabot/docker/update_checker_spec.rb

@@ -185,6 +185,15 @@
       end
     end
 
+    context "when the version is the latest release candidate" do
+      let(:dependency_name) { "php" }
+      let(:tags_fixture_name) { "php.json" }
+      let(:version) { "7.4.0RC6-fpm-buster" }
+      let(:repo_url) { "https://registry.hub.docker.com/v2/library/php/" }
+
+      it { is_expected.to eq("7.4.0RC6-fpm-buster") }
+    end
+
     context "when there is a latest tag" do
       let(:tags_fixture_name) { "ubuntu.json" }
       let(:headers_response) do