Don't override LD_PRELOAD value if already set

Author: anonychun

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

@@ -1,7 +1,7 @@
 #!/bin/bash -e
 
 # Enable jemalloc for reduced memory usage and latency.
-if [ -f /usr/lib/*/libjemalloc.so.2 ]; then
+if [ -z "${LD_PRELOAD+x}" ] && [ -f /usr/lib/*/libjemalloc.so.2 ]; then
   export LD_PRELOAD="$(echo /usr/lib/*/libjemalloc.so.2) $LD_PRELOAD"
 fi