Keep the live mail host out of the repo

Replace the hard-coded mail.inbuxa.com with generic placeholders: the config
default and .env.example use mail.example.com, the README stops naming the QA
host, and docker-compose now requires STALWART_URL to be set rather than
defaulting to somebody's real server.

Nothing deployed depends on the old default - the running container passes
STALWART_URL explicitly.

docker-compose.yml

@@ -6,7 +6,7 @@ services:
     ports:
       - "8080:8080"
     environment:
-      STALWART_URL: ${STALWART_URL:-https://mail.example.com}
+      STALWART_URL: ${STALWART_URL:?set STALWART_URL in .env}
       APP_SECRET: ${APP_SECRET:?set APP_SECRET in .env (openssl rand -base64 48)}
       APP_NAME: ${APP_NAME:-ihasmail}
       TRUST_PROXY: "1"