test_docker_compose.py: Test the volume mount feature

Extend docker_compose_test() to expose /bin on the host to the container
through a volume mount and verify that /bin/busybox can be downloaded and
contains the right data.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/testing/conf/docker-compose.yml

@@ -2,6 +2,8 @@ version: '3'
 services:
   busybox:
     image: "busybox:latest"
-    command: httpd -f -h /etc/
+    command: httpd -f -h /www/
     ports:
       - "80:80"
+    volumes:
+      - "/bin:/www"