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/server.xml

@@ -42,7 +42,7 @@
         name="jdbc/mifosplatform-tenants"
         factory="org.apache.tomcat.jdbc.pool.DataSourceFactory"
         driverClassName="org.drizzle.jdbc.DrizzleDriver"
-        url="jdbc:mysql:thin://fineractmysql:3306/mifosplatform-tenants"
+        url="${fineract_tenants_url}"
         username="root"
         password="mysql"
         initialSize="3"