Wait for healthy containers in integration test

Rather than waiting 5 seconds and hoping for the best after we boot
docker compose, add docker healthchecks and wait for all the containers
to be healthy.

test/integration/docker/load_balancer/Dockerfile

@@ -2,3 +2,4 @@ FROM nginx:1-alpine-slim
 
 COPY default.conf /etc/nginx/conf.d/default.conf
 
+HEALTHCHECK --interval=1s CMD pgrep nginx