[v2.9.0.beta4] Bump default base image
Bump default base image (https://github.com/discourse/discourse_docker/commit/2a9faf7e5680b9979585be613162c45a0960a525)
Bump default base image (https://github.com/discourse/discourse_docker/commit/2a9faf7e5680b9979585be613162c45a0960a525)