docs: pass -T when the env file arrives on stdin

The documented check reads the file from stdin, but `docker compose run`
asks for a TTY unless told not to. Older Compose versions then fail with
"the input device is not a TTY" and the operator never sees the report --
which is the one command this feature exists to offer.

Newer versions detect the redirect and work either way, so the omission
survives a local test and only shows up on the server.

Author: rabbitson87

.env.example

@@ -7,7 +7,10 @@
 # effective value and source of every key, names any key that nothing reads, and
 # says which features are off and what is missing:
 #
-#   docker compose run --rm portal config --env-file /dev/stdin < .env
+#   docker compose run --rm -T portal config --env-file /dev/stdin < .env
+#
+# -T because the file arrives on stdin. Without it Compose asks for a TTY and
+# older versions fail with "the input device is not a TTY".
 #
 # API_PORT and SNI_PORT are deliberately absent. The bundled topology fixes them
 # at 4017 and 443 because the relay reaches its own API listener through its SNI