Fix parsing ruby version for Dockerfile packages

Previously, the version regular expressions never matched
Gem.ruby_version because Gem::Version is not a string. In addition,
ruby_version was undefined and this was not caught before because the
case statement never evaluated those branches.

This commit fixes both of those issues, so that ruby versions like 2.7.0
will correctly install "python" instead of "python-is-python3"

railties/lib/rails/generators/app_base.rb

@@ -520,11 +520,11 @@ def dockerfile_build_packages
           # how python is installed with the bullseye release.  Below
           # is based on debian release included with the Ruby images on
           # Dockerhub.
-          case Gem.ruby_version
-          when /^2.7/
-            bullseye = ruby_version >= "2.7.4"
-          when /^3.0/
-            bullseye = ruby_version >= "3.0.2"
+          case Gem.ruby_version.to_s
+          when /^2\.7/
+            bullseye = Gem.ruby_version >= "2.7.4"
+          when /^3\.0/
+            bullseye = Gem.ruby_version >= "3.0.2"
           else
             bullseye = true
           end

railties/test/generators/app_generator_test.rb

@@ -1047,6 +1047,17 @@ def test_dockerfile
     end
   end
 
+  def test_old_rubies_do_not_use_bullseye_python
+    Gem.stub(:ruby_version, Gem::Version.new("2.7.0")) do
+      run_generator [destination_root, "--js=esbuild"]
+    end
+
+    assert_file "Dockerfile" do |content|
+      assert_match(/python/, content)
+      assert_no_match(/python-is-python3/, content)
+    end
+  end
+
   def test_skip_docker
     run_generator [destination_root, "--skip-docker"]