diff --git a/Dockerfile b/Dockerfile
index 8f987462..542343ae 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -29,8 +29,12 @@ RUN sed -i 's|^run/main_scene=.*$|run/main_scene="res://scenes/server_boot.tscn"
  && mkdir -p /opt/cosmic-clash \
  && godot --headless --path Game --export-release "Linux Dedicated Server" /opt/cosmic-clash/CosmicClashServer.x86_64
 
-# ubuntu:24.04 multi-architecture index, resolved 2026-08-29.
-FROM --platform=linux/amd64 ubuntu@sha256:571c2ab10651ab3a703fcfcb1b06545f5b53085872dcdf68bed17dd7ef4d72db AS server
+# ubuntu:24.04 multi-architecture index, resolved 2026-09-01. The previous
+# pin (571c2ab1...) no longer resolves from Docker Hub as of this date --
+# `docker pull` by that exact digest returns "not found", meaning
+# make verify-phase6 (and this Dockerfile generally) was currently broken for
+# anyone building from a clean cache. Re-pinned to a digest verified to pull.
+FROM --platform=linux/amd64 ubuntu@sha256:33ceb71981b602c1a7443a53469e4dba065f7503eab3078a2d7a57a2ab987517 AS server
 RUN apt-get update && apt-get install -y --no-install-recommends libfontconfig1 libgl1 libstdc++6 && rm -rf /var/lib/apt/lists/*
 COPY --from=exporter /opt/cosmic-clash/ /opt/cosmic-clash/
 COPY deploy/cosmic-clash-server /opt/cosmic-clash/cosmic-clash-server