Just testing it and yes, it fails.
Something must have changed since then.
The fluentd dockerhub page shows how to do this and they switch user to root and back to fluent, maybe this is why it's failing.
This one is building but I didn't tested it:
FROM fluent/fluentd:v1.12-debian
USER root
RUN ["gem", "install", "fluent-plugin-elasticsearch"]
RUN ["gem", "install", "fluent-plugin-input-gelf"]
USER fluent
Can you open an issue on Quarkus repo ?
If you plan to fix it, also update the Dockerfile inside our integration test so we can have it somewhere for testing.