Rename template folder to avoid . prefix

The gemspec doesn't include files with the `.` prefix, so the released
gem didn't include the devcontainer templates. This commit renames the
folder to avoid the `.` prefix, following the same pattern as all
the other `.` templates.

Author: rafaelfranca

railties/lib/rails/generators/rails/devcontainer/devcontainer_generator.rb

@@ -31,9 +31,9 @@ class DevcontainerGenerator < Base # :nodoc:
       def create_devcontainer
         empty_directory ".devcontainer"
 
-        template ".devcontainer/devcontainer.json"
-        template ".devcontainer/Dockerfile"
-        template ".devcontainer/compose.yaml"
+        template "devcontainer/devcontainer.json", ".devcontainer/devcontainer.json"
+        template "devcontainer/Dockerfile", ".devcontainer/Dockerfile"
+        template "devcontainer/compose.yaml", ".devcontainer/compose.yaml"
       end
 
       def update_application_system_test_case