In production ".env.${MODE}" file is not taken in account

Hi,

Just noticed that when going to production (`ssr:build` and `ssr:start`) in the `entry-server.js` I'm able to access all environment variables from `.env` but not those from `.env.${MODE}` (with MODE being your custom mode).

Note that I'am specifying the `--mode` when building and starting the app.

Thank you,