ci(linux/build): self-host the builder image build + iter16 reprepro wrap (M1.1)

Two coupled changes that unblock the M1.1 iter loop. Both belong in CI;
iter1-15 was wrong to require human-in-the-loop steps to make progress.

1. **CI now builds Dockerfile.builder.**

   `.gitea/workflows/build-iso-linux.yaml` grows a `builder-image` job
   that runs ahead of `build-and-verify`. It rebuilds the silvermetal-
   builder image from `linux/build/docker/Dockerfile.builder`, pushes it
   to `docker-registry.silverlabs.uk/silvermetal-builder:m1.1-<sha>` (and
   `:latest`), reads the resulting digest off `docker inspect`, and
   feeds it forward as a job output. `build-and-verify` consumes that
   digest as the `BUILDER_IMAGE` env override that `build.sh` already
   honours (and validates is digest-form on line ~37).

   That kills the old workflow where every Dockerfile.builder change
   required a human to `docker build` + `docker push` on 10.0.0.51 by
   hand and then bump the digest in `build.sh` in lockstep. The crash
   that triggered this (exit 126 mid-iter16 build run) was a symptom of
   that off-CI step still existing.

   Both jobs run on the existing `silvermetal-builder` runner; the host
   docker daemon is shared via DooD and is already authenticated to
   `docker-registry.silverlabs.uk` (linux/build/runner/docker-compose.yml
   mounts `/root/.docker:/root/.docker:ro`), so no extra login step.

   The hardcoded `BUILDER_IMAGE` digest in `build.sh` stays as the
   local-developer / offline-rebuild fallback. Comments updated in
   `build.sh`, `Dockerfile.builder`, and `linux/build/README.md` to
   match the new flow.

2. **reprepro wrapper for the benign "No priority for X" case.**

   Pinned derivative-maker's `2100_create-debian-packages` (with
   --target iso) re-imports source packages from snapshot.debian.org
   into a local apt repo via `reprepro --basedir … includedsc local
   <foo>.dsc`. The local repo's `conf/distributions` ships no
   `DscOverride` entries, so any source package whose `.dsc` lacks an
   explicit Priority field trips:

       No priority for 'X', skipping.
       There have been errors!

   …and reprepro exits 255. dm-reprepro-wrapper bubbles that up,
   2100_create-debian-packages aborts. The current offender is
   `virtualbox_*.dsc` (key import is now fine — debian-keyring landed in
   commit 4aa59ba — but the priority field gap remains). VirtualBox is
   not in SilverMetal's `--target iso` set, so the sane behaviour is
   "log it, continue".

   New `linux/build/docker/silvermetal-reprepro-wrap.sh` shadows
   `/usr/bin/reprepro` at `/usr/local/bin/reprepro` (PATH precedence).
   It runs the real reprepro, captures merged stdout+stderr, and:
   - if rc != 0 AND every non-blank output line matches one of the
     known-benign patterns ("No priority for 'X', skipping." plus the
     trailing "There have been errors!"), emits the output, logs one
     line of explanation to stderr, and exits 0;
   - otherwise emits the output and propagates rc unchanged.

   Any *other* reprepro error path stays fatal — only the specific
   "No priority for X" pattern is neutralised. `dm-reprepro-wrapper`
   resolves `reprepro` via `\$PATH` so it picks up the wrapper
   transparently.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-iso-linux.yaml

@@ -4,6 +4,15 @@ name: Build SilverMetal Linux ISO (reproducibility-gated)
 # isolated directories and gates on byte-identical SHA256. On a tag push, the
 # verified ISO and its SHA256SUMS are attached to a Gitea release.
 #
+# Two-stage:
+#   1. builder-image — rebuilds linux/build/docker/Dockerfile.builder, pushes
+#      to docker-registry.silverlabs.uk/silvermetal-builder:m1.1-<sha>, and
+#      surfaces the resulting digest as a job output. This is what previously
+#      had to be done by hand on 10.0.0.51 between every iter.
+#   2. build-and-verify — reproducibility-gated double build, using the
+#      digest from step 1 via the BUILDER_IMAGE env override that build.sh
+#      already supports (and validates is digest-form).
+#
 # The release-upload pattern (create-if-not-exists then attach asset) is
 # lifted from SilverLABS/SilverVPN/.gitea/workflows/build-linux-client.yaml
 # lines 77-117. Keep them in sync if either changes.
@@ -32,11 +41,77 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  builder-image:
+    # Build + push the silvermetal-builder image on the runner host's docker
+    # daemon (DooD via /var/run/docker.sock, mounted into the act_runner job
+    # container by linux/build/runner/docker-compose.yml). The runner host
+    # is also already authenticated to docker-registry.silverlabs.uk via
+    # /root/.docker (mounted read-only into the runner) so `docker push`
+    # works without an explicit login step here.
+    runs-on: silvermetal-builder
+    timeout-minutes: 30
+    outputs:
+      digest: ${{ steps.push.outputs.digest }}
+      image:  ${{ steps.push.outputs.image }}
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        # No submodules needed for the builder image — its build context is
+        # only linux/build/docker/.
+
+      - name: Build & push silvermetal-builder
+        id: push
+        env:
+          REGISTRY: docker-registry.silverlabs.uk
+          REPO: silvermetal-builder
+        run: |
+          set -eu
+          TAG="m1.1-${GITHUB_SHA::12}"
+          IMAGE="${REGISTRY}/${REPO}:${TAG}"
+          LATEST="${REGISTRY}/${REPO}:latest"
+
+          echo "Building ${IMAGE}"
+          docker build \
+            -f linux/build/docker/Dockerfile.builder \
+            -t "${IMAGE}" \
+            -t "${LATEST}" \
+            linux/build/docker
+
+          echo "Pushing ${IMAGE}"
+          docker push "${IMAGE}"
+          docker push "${LATEST}"
+
+          # docker inspect's RepoDigests is "repo@sha256:...". Take the
+          # entry that matches the registry/repo we just pushed to (there
+          # may be multiple if the image has been pushed elsewhere too).
+          DIGEST=$(docker inspect --format '{{range .RepoDigests}}{{println .}}{{end}}' "${IMAGE}" \
+            | grep "^${REGISTRY}/${REPO}@" \
+            | head -n1 \
+            | sed 's/.*@//')
+          if [ -z "${DIGEST}" ]; then
+            echo "::error::failed to resolve digest for ${IMAGE}" >&2
+            docker inspect "${IMAGE}" >&2 || true
+            exit 1
+          fi
+          echo "Pushed digest: ${DIGEST}"
+
+          {
+            echo "digest=${DIGEST}"
+            echo "image=${REGISTRY}/${REPO}@${DIGEST}"
+          } >> "${GITHUB_OUTPUT}"
+
   build-and-verify:
     # Self-hosted, privileged-capable. Setup procedure documented in
     # linux/build/README.md ("Self-hosted runner setup").
+    needs: builder-image
     runs-on: silvermetal-builder
     timeout-minutes: 240
+    env:
+      # Override build.sh's compiled-in pin with the digest we just built &
+      # pushed. build.sh validates the @sha256: form on line ~37 — the
+      # composed value below satisfies that.
+      BUILDER_IMAGE: ${{ needs.builder-image.outputs.image }}
 
     steps:
       - name: Checkout (with submodules)
@@ -50,7 +125,8 @@ jobs:
           set -eu
           echo "commit=$(git rev-parse HEAD)"
           cat linux/build/config/snapshot-pin.env
-          echo "builder image:"
+          echo "builder image (this run): ${BUILDER_IMAGE}"
+          echo "Dockerfile.builder FROM/snapshot:"
           grep -E '^FROM |^ARG APT_SNAPSHOT_URL' linux/build/docker/Dockerfile.builder
 
       - name: Build A