`envsubst ` distroy the log_format

[yujiaao]

after substitute, the log_format become unusable:

before:

  log_format  main  '$remote_addr $remote_user [$time_local] "$request" '
                '$status "$http_referer" "$http_user_agent"';

after

   log_format  main  '  [] "" '
                ' "" ""';