ci: rebuild prop-base weekly to pull Debian security patches

Adds a Sunday 06:00 UTC schedule trigger to build-base.yaml plus a
CACHE_BUST=<ISO year+week> build-arg keyed off `date -u +%G-W%V`. The
apt RUN steps in Dockerfile.base reference the arg in their command
line so BuildKit's layer hash changes when the week rolls over,
forcing apt-get update + install to re-execute and pull fresh
security updates.

The wgrib2-builder stage doesn't see CACHE_BUST so the ~5-min wgrib2
compile keeps hitting layer cache across weeks; only the apt layers
rebuild. Effective cost of the weekly job: ~2 min for the cdo /
gdal-bin reinstall.

.forgejo/workflows/build-base.yaml

@@ -1,9 +1,13 @@
 name: Build base image
 
-# Path-filtered so this only fires when the base Dockerfile (or this
-# workflow) changes — typically a wgrib2 / g2c version bump or apt-dep
-# tweak. `workflow_dispatch` lets us rebuild on demand without touching
-# the file (e.g. to refresh CVE patches in the apt layer).
+# Path-filtered so it fires when the base Dockerfile (or this workflow)
+# changes, weekly via cron so Debian security updates land without us
+# remembering to bump anything, and on-demand via workflow_dispatch.
+#
+# The weekly cron passes CACHE_BUST=<ISO week> as a build-arg so the
+# apt layers in Dockerfile.base re-execute (otherwise BuildKit's layer
+# cache would short-circuit `apt-get update` on a deterministic input
+# and silently skip security patches).
 on:
   push:
     branches:
@@ -11,6 +15,11 @@ on:
     paths:
       - 'Dockerfile.base'
       - '.forgejo/workflows/build-base.yaml'
+  schedule:
+    # Sundays 06:00 UTC — runs on a quiet day-of-week so a long base
+    # rebuild doesn't collide with mid-week deploys, and matches the
+    # cadence that Debian security advisories typically aggregate at.
+    - cron: '0 6 * * 0'
   workflow_dispatch:
 
 env:
@@ -106,12 +115,19 @@ jobs:
         run: |
           IMAGE="${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}"
           TAG="${{ steps.tag.outputs.tag }}"
+          # ISO year+week (e.g. 2026-W18). Same value all week, so a
+          # cron-triggered rebuild on day 1 vs day 7 reuses the layer
+          # cache; the next ISO week boundary forces apt-get update +
+          # install to re-execute and pull fresh security patches.
+          CACHE_BUST=$(date -u +%G-W%V)
+          echo "CACHE_BUST=${CACHE_BUST}"
 
           attempt=1
           max_attempts=3
           while : ; do
             if docker buildx build \
                 -f Dockerfile.base \
+                --build-arg CACHE_BUST="${CACHE_BUST}" \
                 -t "${IMAGE}:${TAG}" \
                 -t "${IMAGE}:latest" \
                 --push \

Dockerfile.base

@@ -70,12 +70,21 @@ RUN wget -q --content-disposition "https://github.com/NOAA-EMC/wgrib2/archive/re
 # ---- Final runtime base ----
 FROM ${RUNNER_IMAGE} AS base
 
+# Weekly cron passes CACHE_BUST=<iso year+week> so the apt layers below
+# re-execute and pick up Debian security patches. Source-controlled
+# 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.
+ARG CACHE_BUST=none
+
 # Runtime apt deps. cdo + gdal-bin are split into a second RUN so a
 # transient cdo install failure doesn't invalidate the smaller base
-# layer above.
+# layer above. Referencing `${CACHE_BUST}` in the RUN line changes the
+# layer hash week-to-week without altering the install set.
 RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
     --mount=type=cache,target=/var/lib/apt,sharing=locked \
-    rm -f /etc/apt/apt.conf.d/docker-clean \
+    echo "cache-bust=${CACHE_BUST}" \
+    && rm -f /etc/apt/apt.conf.d/docker-clean \
     && apt-get update \
     && apt-get install -y --no-install-recommends \
        libstdc++6 openssl libncurses6 locales ca-certificates snmp \
@@ -83,7 +92,8 @@ RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
 
 RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
     --mount=type=cache,target=/var/lib/apt,sharing=locked \
-    rm -f /etc/apt/apt.conf.d/docker-clean \
+    echo "cache-bust=${CACHE_BUST}" \
+    && rm -f /etc/apt/apt.conf.d/docker-clean \
     && apt-get update \
     && apt-get install -y --no-install-recommends cdo gdal-bin