supply chain: verify signatures and pins across image + kernel builds

Three independent hardenings, addressing a review finding that the
kernel and image build pipelines were relying on HTTPS alone for
artifact integrity.

scripts/make-generic-kernel.sh
- Fetch the detached PGP signature (linux-<ver>.tar.sign) alongside
  the tarball and verify it with gpg before extraction. An isolated
  $GNUPGHOME under the tempdir keeps the kernel signers out of the
  invoking user's keyring.
- Import the three kernel.org release signing keys (Greg KH / Linus /
  Sasha Levin) from keyserver.ubuntu.com, falling back to
  keys.openpgp.org. Ubuntu comes first because keys.openpgp.org strips
  unverified UIDs on upload, leaving gpg with UID-less keys it
  refuses to trust.
- Require VALIDSIG (cryptographic proof) rather than GOODSIG
  (printed even for expired keys) before proceeding. Verified
  end-to-end against a clean tarball (accepts) and a byte-flipped
  tampered copy (rejects with BADSIG).
- gpg + gpgv + xz added to the required-tools check.

images/golden/Dockerfile
- Pin Docker's apt signing key by fingerprint. After downloading
  /etc/apt/keyrings/docker.asc we gpg --show-keys --with-colons it,
  extract the fpr, and compare against the expected
  9DC858229FC7DD38854AE2D88D81803C0EBFCD88. A tampered or swapped key
  aborts the build before any apt repo metadata is fetched.
- Replace `curl https://mise.run | sh` with a pinned GitHub release
  binary (mise v2026.4.18, linux-x64) verified against its published
  sha256. Refuses to build on unknown architectures rather than
  silently installing a binary we have no hash for.
- Add gnupg to the ESSENTIAL apt-get install so the fingerprint check
  has gpg available.

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

images/golden/Dockerfile

@@ -17,8 +17,9 @@ ENV DEBIAN_FRONTEND=noninteractive \
 # -------- 1. ESSENTIAL --------
 # Banger needs: an init (systemd + udev + dbus), sshd (the only
 # control channel), TLS roots + curl (first-boot installs + mise
-# installer), iproute2 (debugging; `ip` is still useful even when
-# the kernel sets IP via cmdline).
+# installer), gnupg (build-time signing-key verification for the
+# Docker apt repo), iproute2 (debugging; `ip` is still useful even
+# when the kernel sets IP via cmdline).
 #
 # udev is a Recommends of the systemd package on Debian. With
 # --no-install-recommends it's skipped — and without it systemd never
@@ -33,6 +34,7 @@ RUN apt-get update \
       openssh-server \
       ca-certificates \
       curl \
+      gnupg \
       iproute2 \
  && rm -rf /var/lib/apt/lists/*
 
@@ -55,25 +57,57 @@ RUN apt-get update \
 # Docker CE (with Compose v2 + buildx) from the official apt repo.
 # Nested-VM docker gives Compose workflows hostname/port isolation
 # per banger VM, which is a big part of the sandbox story.
-RUN install -m 0755 -d /etc/apt/keyrings \
- && curl -fsSL https://download.docker.com/linux/debian/gpg -o /etc/apt/keyrings/docker.asc \
- && chmod a+r /etc/apt/keyrings/docker.asc \
- && printf 'deb [arch=%s signed-by=/etc/apt/keyrings/docker.asc] https://download.docker.com/linux/debian bookworm stable\n' \
-      "$(dpkg --print-architecture)" > /etc/apt/sources.list.d/docker.list \
- && apt-get update \
- && apt-get install -y --no-install-recommends \
+#
+# The apt key is verified against its published fingerprint before
+# we commit it to the signed-by keyring, so a tampered download (or
+# a TLS compromise against download.docker.com) cannot silently
+# swap in an attacker-controlled signing key. Fingerprint source:
+#   https://docs.docker.com/engine/install/debian/#install-using-the-repository
+RUN set -eu; \
+    expected_fpr=9DC858229FC7DD38854AE2D88D81803C0EBFCD88; \
+    install -m 0755 -d /etc/apt/keyrings; \
+    curl -fsSL https://download.docker.com/linux/debian/gpg -o /tmp/docker.asc; \
+    got="$(gpg --with-colons --show-keys --fingerprint /tmp/docker.asc | awk -F: '/^fpr:/ {print $10; exit}')"; \
+    if [ "$got" != "$expected_fpr" ]; then \
+      echo "docker apt key fingerprint mismatch: got $got, want $expected_fpr" >&2; \
+      exit 1; \
+    fi; \
+    mv /tmp/docker.asc /etc/apt/keyrings/docker.asc; \
+    chmod a+r /etc/apt/keyrings/docker.asc; \
+    printf 'deb [arch=%s signed-by=/etc/apt/keyrings/docker.asc] https://download.docker.com/linux/debian bookworm stable\n' \
+      "$(dpkg --print-architecture)" > /etc/apt/sources.list.d/docker.list; \
+    apt-get update; \
+    apt-get install -y --no-install-recommends \
       docker-ce docker-ce-cli containerd.io \
-      docker-buildx-plugin docker-compose-plugin \
- && rm -rf /var/lib/apt/lists/*
+      docker-buildx-plugin docker-compose-plugin; \
+    rm -rf /var/lib/apt/lists/*
 
-# mise — per-repo version manager. Installed system-wide so the
-# bashrc activation reaches every shell.
-RUN curl -fsSL https://mise.run | MISE_INSTALL_PATH=/usr/local/bin/mise sh \
- && chmod 0755 /usr/local/bin/mise \
- && install -d /etc/profile.d \
- && printf '%s\n' 'if [ -x /usr/local/bin/mise ]; then eval "$(/usr/local/bin/mise activate bash)"; fi' \
-      > /etc/profile.d/mise.sh \
- && chmod 0644 /etc/profile.d/mise.sh
+# mise — per-repo version manager. Installed from a pinned GitHub
+# release asset rather than `curl https://mise.run | sh` so a compromise
+# of the installer endpoint can't silently push arbitrary code into
+# the golden image.
+#
+# Update protocol: bump MISE_VERSION + MISE_SHA256 together. Source
+# for the hash is the `digest` field on the release asset from
+# `gh release view --repo jdx/mise --json assets`, or compute from
+# the downloaded file and cross-reference against SHASUMS256.txt on
+# the release page.
+ARG MISE_VERSION=v2026.4.18
+ARG MISE_SHA256_AMD64=6ae2d5f0f23a2f2149bc5d9bf264fe0922a1da843f1903e453516c462b23cc1f
+RUN set -eux; \
+    arch="$(dpkg --print-architecture)"; \
+    if [ "$arch" != "amd64" ]; then \
+      echo "mise pin only tracks amd64; add a ${arch} hash to refresh" >&2; \
+      exit 1; \
+    fi; \
+    curl -fsSL -o /tmp/mise "https://github.com/jdx/mise/releases/download/${MISE_VERSION}/mise-${MISE_VERSION}-linux-x64"; \
+    echo "${MISE_SHA256_AMD64}  /tmp/mise" | sha256sum -c -; \
+    install -m 0755 /tmp/mise /usr/local/bin/mise; \
+    rm /tmp/mise; \
+    install -d /etc/profile.d; \
+    printf '%s\n' 'if [ -x /usr/local/bin/mise ]; then eval "$(/usr/local/bin/mise activate bash)"; fi' \
+      > /etc/profile.d/mise.sh; \
+    chmod 0644 /etc/profile.d/mise.sh
 
 # Default branch for any git init inside the sandbox.
 RUN git config --system init.defaultBranch main