There is just one warning in the logs, before the DNS IPv6 error is reported.
root@basecamp /srv/sentry # docker-compose rm -s -v relay
Going to remove sentry_onpremise_relay_1
Are you sure? [yN] y
Removing sentry_onpremise_relay_1 ... done
root@basecamp /srv/sentry # docker-compose up -d relay
dosentry_onpremise_redis_1 is up-to-date
sentry_onpremise_zookeeper_1 is up-to-date
sentry_onpremise_kafka_1 is up-to-date
Creating sentry_onpremise_relay_1 ... done
root@basecamp /srv/sentry # docker-compose logs -f relay
Attaching to sentry_onpremise_relay_1
relay_1 | 2021-01-07T12:06:47Z [actix::actors::resolver] WARN: Can not create system dns resolver: io error
relay_1 | 2021-01-07T12:06:47Z [relay_server::actors::upstream] ERROR: authentication encountered error: could not send request to upstream
relay_1 | caused by: could not send request using actix-web client
relay_1 | caused by: Failed to connect to host: Failed resolving hostname: no record found for name: web type: AAAA class: IN
relay_1 | caused by: Failed resolving hostname: no record found for name: web type: AAAA class: IN
relay_1 | caused by: Failed resolving hostname: no record found for name: web type: AAAA class: IN