Install the image's Ubuntu packages from the normal mirrors (closes #190)

The image's `ca-certificates`, `nix-bin` and `runit` now come from Ubuntu's archive instead of a dated snapshot, whose service had failed image builds. apt fetches them over plain HTTP and checks each against the archive's signed package lists, so no CA certificates are borrowed from the Go image. The README, `SPEC.md` and `docs/image.md` say so; the amd64-only limit, which came from the removed hash check, is gone.

Rule cut against: `REPO_POLICIES.md` pins every external reference by hash; sneak's ruling at sneak/prompts#72 (comment) allows Ubuntu's own package index.

Model: deepseek-v4-pro (implementation), opus-5-5 (rework)

Dockerfile

@@ -133,32 +133,11 @@ RUN mkdir /etc/sysusers.d \
     && echo 'u  _runit-log  -  "runit svlogd user"  /nonexistent  /sbin/nologin' \
         > /etc/sysusers.d/runit.conf
 
-# ca-certificates, nix-bin and runit, from Ubuntu's archive as it was at
-# the snapshot moment, which is never earlier than the Ubuntu image above.
-# apt checks every package against the snapshot's InRelease files, and
-# this step checks those against the hashes named here, which are those
-# of the amd64 archive: other architectures use Ubuntu's ports archive.
-# apt also fetches the live archive's InRelease files, which change daily
-# and which the install does not use. The snapshot service is HTTPS only
-# and this image has no CA certificates yet, so this step uses the Go
-# image's. The snapshot service answers 503 at times; apt retries a
-# failed download, waiting twice as long before each retry, and this
-# step gives it five retries, about half a minute in all, where its
-# default of three gives up after seven seconds.
-RUN --mount=type=bind,from=builder,source=/etc/ssl/certs/ca-certificates.crt,target=/tmp/go-image-ca.crt \
-    apt-get update --snapshot 20261001T000000Z \
-        -o Acquire::https::CaInfo=/tmp/go-image-ca.crt \
-        -o Acquire::Retries=5 \
-    && printf '%s\n' \
-        '45f95ce276cdba3e41870516a130e03c58b8b7a79e9546b0efe9e526d255740c  snapshot.ubuntu.com_ubuntu_20261001T000000Z_dists_resolute_InRelease' \
-        '802e675dd9de4c7f3916434a95e7c1d8eec0e82886622d7805ab19a2c6fe0365  snapshot.ubuntu.com_ubuntu_20261001T000000Z_dists_resolute-updates_InRelease' \
-        '64b3353f0bd4970b4f7271962245bcea9ff24d4cc7bea16b433f8a60e42ca3dd  snapshot.ubuntu.com_ubuntu_20261001T000000Z_dists_resolute-backports_InRelease' \
-        '1d5041572116a8b23aabf79ac7439ad8af83d57ad3fb0f9aa0d4523ec10c5908  snapshot.ubuntu.com_ubuntu_20261001T000000Z_dists_resolute-security_InRelease' \
-        | (cd /var/lib/apt/lists && sha256sum --check --strict) \
+# ca-certificates, nix-bin and runit, from Ubuntu's archive. apt fetches
+# them over plain HTTP and checks each one against the archive's signed
+# package lists, so no CA certificates are needed.
+RUN apt-get update \
     && DEBIAN_FRONTEND=noninteractive apt-get install --yes --no-install-recommends \
-        --snapshot 20261001T000000Z \
-        -o Acquire::https::CaInfo=/tmp/go-image-ca.crt \
-        -o Acquire::Retries=5 \
         ca-certificates nix-bin runit \
     && rm -rf /var/lib/apt/lists/*