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/registry/Dockerfile

@@ -4,4 +4,6 @@ COPY boot.sh .
 
 RUN ln -s /shared/certs /certs
 
+HEALTHCHECK --interval=1s CMD pgrep registry
+
 ENTRYPOINT ["./boot.sh"]