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/shared/boot.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+cp -r * /shared
+
+trap "pkill -f sleep" term
+
+sleep infinity & wait