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>

linux/build/README.md

@@ -102,7 +102,7 @@ Each of these is a deliberate, reviewed action — never automate:
 
 - **`derivative-maker` submodule** — bump in its own PR, with a verification log showing two clean builds match.
 - **`snapshot-pin.env`** — same procedure.
-- **Builder image (`Dockerfile.builder` digest)** — rebuild, push, update `BUILDER_IMAGE` in `build.sh`, run reproducibility check, commit all four together.
+- **Builder image (`Dockerfile.builder`)** — edit and commit. CI's `builder-image` job rebuilds, pushes, and feeds the new digest to `build-and-verify` automatically; no manual `docker build`/`docker push` step. The hardcoded `BUILDER_IMAGE` digest fallback in `build.sh` is for local/offline rebuilds only — bump it opportunistically after any merged Dockerfile change so non-CI `build.sh` keeps working at that commit.
 
 ## What this milestone is *not*
 

linux/build/docker/Dockerfile.builder

@@ -19,17 +19,15 @@
 # of the reproducibility gate, so do NOT replace the FROM line with a
 # tag-only reference.
 #
-# Build & push (run on 10.0.0.51 — never on the WSL/aarch64 dev box):
-#   docker build \
-#     -f linux/build/docker/Dockerfile.builder \
-#     -t docker-registry.silverlabs.uk/silvermetal-builder:<commit> \
-#     -t docker-registry.silverlabs.uk/silvermetal-builder:latest \
-#     linux/build/docker
-#   docker push docker-registry.silverlabs.uk/silvermetal-builder:<commit>
-#
-# To bump the base image: replace the digest, rebuild, push, update
-# BUILDER_IMAGE in linux/build/scripts/build.sh, run a full reproducibility
-# check, commit all the changes together.
+# Build & push: handled by .gitea/workflows/build-iso-linux.yaml
+# (`builder-image` job). Every push that touches linux/** rebuilds this
+# Dockerfile on the silvermetal-builder runner, pushes it as
+# docker-registry.silverlabs.uk/silvermetal-builder:m1.1-<sha> + :latest,
+# and feeds the resulting digest into the build-and-verify job via the
+# BUILDER_IMAGE env override that build.sh supports. Do NOT build it
+# locally during normal iter cycles — let CI do it. The pin in
+# build.sh#BUILDER_IMAGE is only the local-developer / offline-rebuild
+# fallback.
 
 # debian:trixie-slim — pinned by digest.
 # Resolved 2026-05-07 via `docker pull debian:trixie-slim` on 10.0.0.51.
@@ -134,5 +132,12 @@ COPY systemd-entrypoint/docker-entrypoint.target       /etc/systemd/system/docke
 COPY systemd-entrypoint/docker-entrypoint-stop.sh      /usr/bin/docker-entrypoint-stop.sh
 RUN chmod +x /usr/local/bin/entrypoint.sh /usr/bin/docker-entrypoint-stop.sh
 
+# reprepro wrapper that translates the benign "No priority for X" errors
+# to exit 0. /usr/local/bin precedes /usr/bin in $PATH, so this masks
+# the real reprepro for everything that respects PATH (including
+# dm-reprepro-wrapper). See the script header for the full rationale.
+COPY silvermetal-reprepro-wrap.sh /usr/local/bin/reprepro
+RUN chmod +x /usr/local/bin/reprepro
+
 ENTRYPOINT ["/usr/local/bin/entrypoint.sh"]
 CMD ["/bin/bash"]

linux/build/docker/silvermetal-reprepro-wrap.sh

@@ -0,0 +1,62 @@
+#!/bin/bash
+# SilverMetal reprepro wrapper — installed at /usr/local/bin/reprepro,
+# masking /usr/bin/reprepro for callers that respect $PATH.
+#
+# Why this exists
+# ---------------
+# The pinned derivative-maker tag includes 2100_create-debian-packages,
+# which (when --target iso) re-imports Debian source packages from
+# snapshot.debian.org into a local apt repo via:
+#     reprepro --basedir … includedsc local <some>.dsc
+# The local repo's conf/distributions doesn't ship DscOverride entries,
+# so any source package whose .dsc lacks an explicit Priority field
+# trips reprepro's:
+#     No priority for 'X', skipping.
+#     There have been errors!
+# and reprepro exits 255. dm-reprepro-wrapper bubbles the exit code up
+# and 2100_create-debian-packages aborts.
+#
+# This is the situation for virtualbox_*.dsc in particular (key import
+# now passes thanks to debian-keyring, but the priority issue remains).
+# The package isn't actually used by SilverMetal's --target iso build —
+# so the right downstream behaviour is "log it, but proceed".
+#
+# What this wrapper does
+# ----------------------
+# Run the real reprepro, capture its merged output, and:
+#   * if rc != 0 AND every non-blank output line matches one of the
+#     known-benign patterns (currently just "No priority for X,
+#     skipping." plus the trailing "There have been errors!"),
+#     emit the output verbatim, log a single line to stderr, and
+#     exit 0.
+#   * otherwise emit the output and propagate reprepro's exit code
+#     unchanged.
+#
+# This keeps any *other* reprepro error fatal — we only neutralise
+# the specific "No priority for X" path.
+
+set -uo pipefail
+
+REAL=/usr/bin/reprepro
+
+# Run real reprepro, capture merged stdout+stderr.
+output=$("$REAL" "$@" 2>&1)
+rc=$?
+
+if [ "$rc" -ne 0 ]; then
+    # Lines that match either of the known-benign patterns are filtered
+    # out; whatever remains is the *real* error surface.
+    remaining=$(printf '%s\n' "$output" \
+        | grep -vE "^No priority for '[^']+', skipping\.$" \
+        | grep -vE '^There have been errors!$' \
+        | grep -vE '^$' || true)
+
+    if [ -z "$remaining" ]; then
+        printf '%s\n' "$output"
+        printf '\n[silvermetal-reprepro-wrap] Only benign "No priority for" errors; treating exit %d as 0.\n' "$rc" >&2
+        exit 0
+    fi
+fi
+
+printf '%s\n' "$output"
+exit "$rc"

linux/build/scripts/build.sh

@@ -25,8 +25,14 @@ REPO_ROOT="$(cd -- "${SCRIPT_DIR}/../../.." && pwd)"
 cd "${REPO_ROOT}"
 
 # --- Pinned builder image ---------------------------------------------------
-# Bumped together with linux/build/docker/Dockerfile.builder. The digest form
-# is required; refusing the tag-only form is what stops a silent host drift.
+# In CI this is always overridden by the BUILDER_IMAGE env var that the
+# `builder-image` job in .gitea/workflows/build-iso-linux.yaml passes in
+# (the digest of the silvermetal-builder image it just built and pushed).
+# The hardcoded default below is the local-developer / offline-rebuild
+# fallback; bump it after any meaningful Dockerfile.builder change merges
+# so `linux/build/scripts/build.sh` works without CI for the same commit.
+# The digest form is required either way; refusing the tag-only form is
+# what stops a silent host drift.
 #
 # docker-registry.silverlabs.uk is the canonical hostname both inside and
 # outside the LAN — it's the entry that fleet-wide /etc/docker/daemon.json