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

The final image stage now apt-gets ca-certificates, nix-bin and runit
from Ubuntu's archive, with no snapshot and no hash check of the package
lists. The borrowed CA bundle is dropped: apt fetches the packages over
plain HTTP and checks each one against the archive's signed package
lists, so the install needs none. The Ubuntu base image stays pinned by
digest, and every other pin is kept. README.md, SPEC.md and
docs/image.md no longer describe a snapshot.

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

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