Adds option to configure docker-compose URL

Necessary for installing from a local mirror or in an offline environmment.

tasks/docker-compose.yml

@@ -15,6 +15,6 @@
 
 - name: Install Docker Compose (if configured).
   get_url:
-    url: https://github.com/docker/compose/releases/download/{{ docker_compose_version }}/docker-compose-Linux-x86_64
+    url: "{{ docker_compose_url }}"
     dest: "{{ docker_compose_path }}"
     mode: 0755