Tag the image build in the checklist's script/cibuild item (closes #125)

The new-repository checklist gave script/cibuild's image build without a
tag, so a repository written from it left a dangling image behind on
every run, and it used $version without saying where it comes from. The
item now gives the build as the canonical script/cibuild runs it, with
-t "$tag", and names the two steps the script runs before it, each on
its own line: the version from git describe --tags --always --dirty
(unknown if empty) and the tag from script/projectname. The other docker
build commands the checklists and REPO_POLICIES.md give for script/
already matched their scripts.

Model: opus-5-5

prompts/NEW_REPO_CHECKLIST.md

@@ -164,9 +164,12 @@ are thin shims calling them. Model scripts:
       version as a build arg
 - [ ] `script/cibuild` — cd to repo root, run `script/bootstrap`, run
       `script/check`, then
-      `docker build --no-cache --build-arg VERSION="$version" .` (what CI runs).
-      The bootstrap is required: CI checks out and runs this alone, and
-      `script/fmt-check` runs the formatter on the host.
+      `docker build --no-cache --build-arg VERSION="$version" -t "$tag" .` (what
+      CI runs), with `$version` from `git describe --tags --always --dirty`
+      (`unknown` if empty) and `$tag` from `script/projectname`, each assigned
+      on its own line before the build. The bootstrap is required: CI checks out
+      and runs this alone, and `script/fmt-check` runs the formatter on the
+      host.
 - [ ] `script/fmt` and `script/fmt-check` source nvm for the pinned node version
       before invoking `yarn`, as `script/bootstrap`'s own install step does.
       `script/bootstrap` leaves the node and yarn it installs off the `PATH` of