Allow customizing JDBC URLs at run-time instead of during container build (FINERACT-773)

In Docker, set JAVA_OPTS instead of FINERACT_TENANTS_URL.

Because ARG/ENV in the Dockerfile can't (easily) be changed at run-time.

At first I tried to override the ENTRYPOINT with a script which sets
JAVA_OPTS based on FINERACT_TENANTS_URL and then delegates to the original
/app-entrypoint.sh; that didn't immediately work, so this seems simpler.

Including make default tenant JDBC hostname & port configurable at Docker container run instead of build time.

docker-compose.yml

@@ -42,3 +42,7 @@ services:
       - 8443:8443
     depends_on:
       - fineractmysql
+    environment:
+      - JAVA_OPTS=-Dfineract_tenants_url=jdbc:mysql:thin://fineractmysql:3306/mifosplatform-tenants -Djava.awt.headless=true -XX:+UseG1GC -Dfile.encoding=UTF-8
+      - FINERACT_DEFAULT_TENANTDB_HOSTNAME=fineractmysql
+      - FINERACT_DEFAULT_TENANTDB_PORT=3306