Stop the lint and test builds writing an image (closes #123)

script/lint and script/test build their Dockerfile phase with
--output type=cacheonly in place of a tag. The phase still runs
uncached and a failing step still fails the build, but no image is
written: nothing used those images, and writing one out took about 16
seconds of a Go repository's test build. script/cibuild and
script/docker keep their tags. REPO_POLICIES.md, both checklists and
the README now say the gate builds write no image.

Model: opus-5-5

README.md

@@ -116,10 +116,14 @@ alpine. We provide:
   `script/bootstrap`, then `script/install-precommit`
 - `script/projectname` — output the project name (our own extension); used by
   `script/docker` for the image tag
-- `script/test` — `docker build --no-cache --target test -t prompts-test .`,
-  building the `test` phase of the `Dockerfile` (no tests defined here)
-- `script/lint` — `docker build --no-cache --target lint -t prompts-lint .`,
-  building the `lint` phase, which runs prettier over the markdown files
+- `script/test` —
+  `docker build --no-cache --target test --output type=cacheonly .`, building
+  the `test` phase of the `Dockerfile` without writing an image (no tests
+  defined here)
+- `script/lint` —
+  `docker build --no-cache --target lint --output type=cacheonly .`, building
+  the `lint` phase, which runs prettier over the markdown files, without writing
+  an image
 - `script/fmt` — format all markdown files with prettier (writes; native, not in
   a container)
 - `script/fmt-check` — check formatting (read-only; native)