Add quotations for consistency with other envs

Follow up to rails#55252

Author: morgoth

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

@@ -25,7 +25,7 @@ ENV RAILS_ENV="production" \
     BUNDLE_DEPLOYMENT="1" \
     BUNDLE_PATH="/usr/local/bundle" \
     BUNDLE_WITHOUT="development" \
-    LD_PRELOAD=/usr/local/lib/libjemalloc.so
+    LD_PRELOAD="/usr/local/lib/libjemalloc.so"
 
 # Throw-away build stage to reduce size of final image
 FROM base AS build