Add ability to use different hostnames for stalwart and roundcube

.env.example

@@ -24,6 +24,12 @@ STALWART_RECOVERY_ADMIN=
 ROUNDCUBE_BIND=127.0.0.1
 ROUNDCUBE_PORT=8091
 
+# Optional IMAP endpoint for Roundcube. Set an IP address here when DNS for
+# MAIL_HOSTNAME is unavailable from the Roundcube container.
+# ROUNDCUBE_IMAP_HOST=192.0.2.10
+# Optional SMTP submission endpoint for Roundcube. Defaults to MAIL_HOSTNAME.
+# ROUNDCUBE_SMTP_HOST=192.0.2.10
+
 # Roundcube database
 RC_DB_NAME=roundcube
 RC_DB_USER=roundcube

README.md

@@ -27,6 +27,8 @@ Edit `.env` and set production values, especially:
 
 The real `.env` file is ignored by Git. Do not commit credentials or recovery settings.
 
+Roundcube uses `MAIL_HOSTNAME` for IMAP and SMTP submission by default. To connect Roundcube to Stalwart by IP instead, add `ROUNDCUBE_IMAP_HOST=<ip-address>` and `ROUNDCUBE_SMTP_HOST=<ip-address>` to `.env`. These change only Roundcube's IMAP and SMTP endpoints; `MAIL_HOSTNAME` remains the Stalwart hostname and Docker network alias.
+
 ## Generate a DES key
 
 On Linux or macOS:

docker-compose.yml

@@ -91,10 +91,10 @@ services:
 
       ROUNDCUBEMAIL_DES_KEY: ${RC_DES_KEY}
 
-      ROUNDCUBEMAIL_DEFAULT_HOST: ssl://${MAIL_HOSTNAME}
+      ROUNDCUBEMAIL_DEFAULT_HOST: ssl://${ROUNDCUBE_IMAP_HOST:-${MAIL_HOSTNAME}}
       ROUNDCUBEMAIL_DEFAULT_PORT: 993
 
-      ROUNDCUBEMAIL_SMTP_SERVER: tls://${MAIL_HOSTNAME}
+      ROUNDCUBEMAIL_SMTP_SERVER: tls://${ROUNDCUBE_SMTP_HOST:-${MAIL_HOSTNAME}}
       ROUNDCUBEMAIL_SMTP_PORT: 587
 
       ROUNDCUBEMAIL_USERNAME_DOMAIN: ${MAIL_DOMAIN}