fix(ci): set DD_HOSTNAME in the agent container (#4458) (#4468)

Agent version v7.40.0 introduced a breaking change which will cause the
agent to fail to start if DD_HOSTNAME is not set and the agent cannot
deduce a hostname for the container.

https://github.com/DataDog/datadog-agent/releases/tag/7.40.0 (see the
"Other Notes" section)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit 398f025)

Co-authored-by: Kyle Verhoog <[email protected]>

Author: mergify[bot]

docker-compose.yml

@@ -94,6 +94,7 @@ services:
     ddagent:
         image: datadog/agent:latest
         environment:
+            - DD_HOSTNAME=github-actions-worker
             - DD_BIND_HOST=0.0.0.0
             - DD_API_KEY=${DD_API_KEY-invalid_but_this_is_fine}
             - DD_APM_RECEIVER_SOCKET=/tmp/ddagent/trace.sock