ci: tag the trivy image so the reclaim step stops deleting it

Run #68 re-pulled trivy in all three matrix legs — 178MB each, ~7s each —
because the reclaim step deletes it at the end of every leg:

  Unable to find image 'aquasec/trivy:0.72.0@sha256:cffe...' locally
  untagged: aquasec/trivy@sha256:cffe...
  Total reclaimed space: 178.6MB

The `--filter until=168h` I added for exactly this reason does not work.
That filter reads the image's CREATED timestamp, not the pull time, and
aquasec/trivy:0.72.0 was built months ago, so it matches immediately. The
comment claiming the age filter fixed this was wrong; corrected in place.

Pull by digest, tag it, run the tag. A tagged image is not dangling, which
is what actually takes it out of `docker image prune`'s scope. The digest
is still the pin — enforced at pull time.

Everything else in run #68 is clean: 76 unit (98.59% coverage), 112
integration, 18 CACHED layers per image job, zero uv hardlink warnings,
and the trivy DB volume hit on 2 of 3 legs (one download, two silent).

.gitea/workflows/ci.yaml

@@ -431,11 +431,30 @@ jobs:
         # comment here claimed, so the retention story matters. Trivy replaces the DB in
         # place rather than accumulating versions, so the volume stays at roughly one DB,
         # and the weekly prune CronJob reclaims it at the cost of one re-download.
+        env:
+          TRIVY: aquasec/trivy:0.72.0@sha256:cffe3f5161a47a6823fbd23d985795b3ed72a4c806da4c4df16266c02accdd6f
         run: |
+          # Pull by digest, then give it a local tag, and run the TAG.
+          #
+          # An image pulled by digest carries no tag, which makes it dangling the moment its
+          # container exits — so the reclaim step below deleted it at the end of every matrix
+          # leg and the next leg paid a 178MB re-pull. Three legs, three pulls, every run.
+          #
+          # The `until=168h` filter there does not save it. That filter reads the image's
+          # CREATED timestamp, not when it was pulled, and this image was built months ago,
+          # so it matches the age filter immediately. A tag is what actually takes an image
+          # out of `docker image prune`'s scope. Verified in run #68: "Unable to find image
+          # ... locally" in all three legs, and "untagged: aquasec/trivy@sha256:cffe..." in
+          # each prune.
+          #
+          # The digest is still the pin — it is enforced here, at pull time. `trivy:pinned`
+          # is a local alias for an image whose content was already verified.
+          docker pull "${TRIVY}"
+          docker tag "${TRIVY}" trivy:pinned
           docker run --rm \
             -v /var/run/docker.sock:/var/run/docker.sock \
             -v svcforge-trivy-db:/root/.cache/trivy \
-            aquasec/trivy:0.72.0@sha256:cffe3f5161a47a6823fbd23d985795b3ed72a4c806da4c4df16266c02accdd6f \
+            trivy:pinned \
             image \
               --severity HIGH,CRITICAL \
               --ignore-unfixed \
@@ -482,12 +501,17 @@ jobs:
         #
         # Named volumes are never pruned here: that is where the trivy vuln DB lives.
         #
-        # Both prunes carry an age filter, because "dangling" catches more than it looks.
-        # An image pulled by digest has no tag, so it is dangling the moment its container
-        # exits. A bare `docker image prune -f` therefore deleted the trivy image on every
-        # run and bought back a 157MB pull on the next one. The act runner image survived
-        # that only by accident: this step executes inside an act container, so the image
-        # is in use exactly while the prune runs. Relying on that is not a design.
+        # "Dangling" catches more than it looks: an image pulled by digest has no tag, so it
+        # is dangling as soon as its container exits. That is why the trivy scan step tags
+        # its image — an age filter does NOT protect it, because `until` reads the image's
+        # created time, and a released tool image is always older than any useful window.
+        # Run #68 proved that: trivy was untagged and deleted in all three legs despite
+        # `until=168h`, and re-pulled 178MB each time.
+        #
+        # The age filter stays anyway, for the buildx cache, which really does grow by age.
+        # The act runner image survives only because this step runs inside an act container,
+        # so the image is in use exactly while the prune runs. That is luck, not design; if
+        # it ever starts disappearing, tag it the same way.
         if: always()
         run: |
           docker image prune -f --filter until=168h