compose: Wait until fleet-server is healthy (#7034)

Updates the `proxy_dep` in `docker-compose.yml` to wait until the`fleet-
server` is healthy before exiting. This will most likely cause the
systemtest to fail less on `context.DeadlineExceeded` since the wait
won't count towards the systemtest timeout.

Signed-off-by: Marc Lopez Rubio <[email protected]>

Author: marclop

docker-compose.yml

@@ -7,6 +7,7 @@ services:
     depends_on:
       elasticsearch: { condition: service_healthy }
       kibana: { condition: service_healthy }
+      fleet-server: { condition: service_healthy }
 
   elasticsearch:
     image: docker.elastic.co/elasticsearch/elasticsearch:8.1.0-dbc834fd-SNAPSHOT