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/*
 

README.md

@@ -69,8 +69,7 @@ Not built, and not planned:
 ### The image
 
 No image is published. Build it from a clone. `make docker` runs the tests and
-the linter, then builds the image, tagged `smallwebwaf`. It builds for amd64, on
-an amd64 host only.
+the linter, then builds the image, tagged `smallwebwaf`.
 
 ```sh
 git clone https://git.eeqj.de/sneak/smallwebwaf.git
@@ -662,8 +661,8 @@ aside as `<name>.bad`, and never stops the running process (see
 
 ### The image and its processes
 
-- The image is built on Ubuntu 26.04 LTS, pinned by digest, with its packages
-  from a snapshot of Ubuntu's archive and nixpkgs fixed at one release. An app's
+- The image is built on Ubuntu 26.04 LTS, pinned by digest. Its Ubuntu packages
+  come from Ubuntu's archive, and nixpkgs is fixed at one release. An app's
   `Dockerfile` installs a package with `nix-env -iA nixpkgs.<name>` and gets the
   same package each time it is built (see [`docs/image.md`](docs/image.md)).
 - runit, a service supervisor, runs `smallwebwaf` as the `smallwebwaf` user (uid

SPEC.md

@@ -1544,20 +1544,11 @@ It sets no `ENTRYPOINT` or `USER` of its own. The container must start
 
 - The image is Ubuntu's newest long-term support release, pinned by digest, and
   moves to the next LTS release when that ships. Its packages come from Ubuntu's
-  snapshot service, the archive as it was at the moment the Dockerfile names,
-  rather than the live archive, whose packages change with every update. The
-  snapshot is never older than the pinned Ubuntu image, since older packages can
-  need older versions of packages the image holds, and it moves forward with the
-  image's digest. The Dockerfile names the hash of each of the snapshot's
-  `InRelease` files, which cover every package list and so every package, and
-  the build checks them before installing. The live archive's `InRelease` files,
-  which `apt-get update` also fetches and the install does not use, are left out
-  of the check. The hashes are the amd64 archive's, so the image is built for
-  amd64 only.
+  archive, whose packages change with every update.
 - The Ubuntu image has no CA certificates. Without them Nix cannot download
   packages and `servewaf` cannot reach GeoJS, so `ca-certificates` is installed
-  by name. The snapshot itself is fetched over HTTPS with the CA certificates of
-  the Go image that builds `smallwebwaf`, itself pinned by digest.
+  by name. apt fetches the packages over plain HTTP and checks each one against
+  the archive's signed package lists, so the install needs no CA certificates.
 - nixpkgs comes from a release file on `releases.nixos.org`, named with its
   hash, not from a GitHub archive, whose bytes can change. Root uses Nix
   directly; no Nix daemon runs. `nix-bin` creates no build users, so

docs/image.md

@@ -8,8 +8,7 @@ is pinned that way.
 ## Contents
 
 - Ubuntu 26.04 LTS, pinned by digest.
-- `ca-certificates`, `nix-bin` and `runit` from Ubuntu, from the snapshot of its
-  archive at `20261001T000000Z`.
+- `ca-certificates`, `nix-bin` and `runit` from Ubuntu's archive.
 - Nix, with nixpkgs `nixos-26.05.11045.774debe7a0d1` set up for root as
   `nixpkgs`. No Nix daemon; `build-users-group =` in `/etc/nix/nix.conf`. Root's
   Nix profile, `/nix/var/nix/profiles/default/bin`, last on the `PATH`.
@@ -26,10 +25,8 @@ is pinned that way.
 
 ## Pins
 
-- Ubuntu's packages: the snapshot service, `--snapshot 20261001T000000Z` on both
-  `apt-get update` and `apt-get install`. The SHA-256 hash of each of the
-  snapshot's `InRelease` files is named in the `Dockerfile` and checked after
-  `apt-get update`. The hashes are the amd64 archive's.
+- Ubuntu's packages are not pinned: they come from the live archive, whose
+  packages change with every update.
 - nixpkgs: the release file `nixexprs.tar.xz` of
   `nixos-26.05.11045.774debe7a0d1` on `releases.nixos.org`, named with its
   SHA-256 hash.