Add BOOTSNAP_READONLY to Dockerfile template for production

This was added in back in bootsnap 1.15.0 (Shopify/bootsnap#428).
This can save extra work and IO when the cache won't be changed, like is the case for docker containers

railties/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Change generated Dockerfile to use bootsnap read-only mode. This saves
+    some work of bootsnap trying to update its cache when it won't be re-used.
+
+    *Josh Nichols*
+
 *   Fix running `db:system:change` when app has no Dockerfile.
 
     *Hartley McGuire*

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

@@ -79,6 +79,9 @@ RUN SECRET_KEY_BASE_DUMMY=1 ./bin/rails assets:precompile
 # Final stage for app image
 FROM base
 
+# Switch to read-only since any cache changes after this point won't persist between containers
+ENV BOOTSNAP_READONLY="1"
+
 <% unless dockerfile_deploy_packages.empty? -%>
 # Install packages needed for deployment
 RUN apt-get update -qq && \