Use environment variable for the image tag

Author: toshimaru

compose.yml

@@ -1,8 +1,7 @@
 services:
   app:
-    image: ruby:3.3
-    # Use Ruby 2.7 for older builds
-    # image: ruby:2.7
+    # NOTE: Use Ruby 2.7 for older builds
+    image: ruby:${RUBY_VERSION:-3.3}
     working_dir: /app
     volumes:
       - .:/app