docker-entrypoint: export LD_PRELOAD

Otherwise it won't applied to the execed process.

Fix: rails#50943 (comment)

Author: byroot

railties/lib/rails/generators/rails/app/templates/docker-entrypoint.tt

@@ -2,7 +2,7 @@
 
 # Enable jemalloc for reduced memory usage and latency.
 if [ -f /usr/lib/*/libjemalloc.so.2 ]; then
-  LD_PRELOAD="$(echo /usr/lib/*/libjemalloc.so.2) $LD_PRELOAD"
+  export LD_PRELOAD="$(echo /usr/lib/*/libjemalloc.so.2) $LD_PRELOAD"
 fi
 
 <% unless skip_active_record? -%>