Make it possible for users to customise exposed ports (#182)

* move portmapping into docker-compose.ports.yml

Ensure loading of docker-compose.ports.yml via COMPOSE_FILE in .env.

* instruct users to put overridden ports into override.yml

docker-compose.ports.yml

@@ -0,0 +1,32 @@
+version: "3.5"
+
+services:
+  web:
+    ports:
+      - "${CADDY:-2015}:2015"
+      - "${HTTP:-80}:80"
+      - "${HTTPS:-443}:443"
+
+  ldap:
+    ports:
+      - ${LDAPPORT:-389}:389
+
+  mail:
+    ports:
+      - "${SMTPPORT:-25}:25"
+      - "${SMTPSPORT:-465}:465"
+      - "${MSAPORT:-587}:587"
+
+  kopano_server:
+    ports:
+      - ${KOPANOPORT:-236}:236
+      - ${KOPANOSPORT:-237}:237
+
+  kopano_gateway:
+    ports:
+      - "${POP3PORT:-110}:110"
+      - "${IMAPPORT:-143}:143"
+
+  kopano_ical:
+    ports:
+      - "${ICALPORT:-8080}:8080"