diff --git a/Dockerfile.base b/Dockerfile.base
index ed0a8475..fc59e6c1 100644
--- a/Dockerfile.base
+++ b/Dockerfile.base
@@ -75,6 +75,12 @@ FROM ${RUNNER_IMAGE} AS base
 # pushes leave the value at its `none` default, so day-to-day base
 # rebuilds (Dockerfile.base edits) don't pay the apt-update cost
 # unless the week has rolled over.
+#
+# Note: the registry occasionally GCs base-image blobs that the
+# manifest still references (saw this 2026-05-04 — `final 1/5 FROM
+# prop-base:latest` failed with "could not fetch content descriptor
+# … not found"). When that happens, push any change to this file to
+# re-trigger the build-base workflow and re-upload the layers.
 ARG CACHE_BUST=none
 
 # Runtime apt deps. cdo + gdal-bin are split into a second RUN so a