Added declaration to generate errors when running a Docker build with warnings.

Author: igor-alexandrov

railties/lib/rails/generators/rails/app/templates/Dockerfile.tt

@@ -1,4 +1,5 @@
-# syntax = docker/dockerfile:1
+# syntax=docker/dockerfile:1
+# check=error=true
 
 # This Dockerfile is designed for production, not development. Use with Kamal or build'n'run by hand:
 # docker build -t my-app .