Pull the base images fresh on every build (bau-pm BAU-191)

Finding 41 of bau-pm's v0.22.3 security review called this a pin to move.
It is not a pin: both bases here are floating tags, so what ships is
whatever the runner had cached. v0.1.0 went out with nginx 1.31.6 while
quay.io/nginx/nginx-unprivileged:alpine-slim had already moved to 1.31.8,
past five October CVEs - none of them reachable in a static file server
configured like this one, which is why --pull is the fix rather than a pin.

The CI build gets it too: a gate running against a months-old cached base
is not testing what the release builds.

A release is what applies this. The running image does not change until one
is cut.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -28,7 +28,10 @@ jobs:
       # shared runner; the vendor stage downloads about 60 MB from npm.
       - name: build the image
         timeout-minutes: 60
-        run: docker build --build-arg VERSION="ci-${GITHUB_SHA::8}" -t bau-pm-pdf-tools:ci .
+        # --pull, as in the release workflow (bau-pm BAU-191): both bases are
+        # floating tags, and a gate that runs against a months-old cached base
+        # is not testing what the release builds.
+        run: docker build --pull --build-arg VERSION="ci-${GITHUB_SHA::8}" -t bau-pm-pdf-tools:ci .
 
       - name: build the gate
         timeout-minutes: 15