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

prompts/REPO_POLICIES.md

@@ -118,9 +118,8 @@ style conventions are in separate documents:
   and nothing else:
 
     ```sh
-    tag="$(script/projectname)"
-    docker build --no-cache --target lint -t "$tag-lint" .
-    docker build --no-cache --target test -t "$tag-test" .
+    docker build --no-cache --target lint --output type=cacheonly .
+    docker build --no-cache --target test --output type=cacheonly .
     ```
 
     **A stage that is not the last one in the file is built only when the final
@@ -130,12 +129,15 @@ style conventions are in separate documents:
     plain `docker build .` builds the last stage alone and exits 0 having linted
     and tested nothing.
 
-    **Every `docker build` in `script/` is tagged**, here and in
-    `script/cibuild` and `script/docker`. An untagged build leaves a dangling
-    image behind on every invocation, on every developer host and every CI
-    runner; a tagged one replaces the previous image. Each script assigns the
-    tag on its own line before the build, so `set -e` stops it where
-    `script/projectname` fails.
+    **The gate builds write no image.** With `--output type=cacheonly` the phase
+    runs and a failing step fails the build, but the result is not exported.
+    Nothing uses those images, and writing one out is slow: a Go test phase's
+    image holds the toolchain and every compiled package. A build given neither
+    `--output` nor `-t` writes an untagged image and leaves it dangling, on
+    every developer host and every CI runner. `script/cibuild` and
+    `script/docker` build the image that ships and tag it, so each build
+    replaces the previous image; each assigns the tag on its own line before the
+    build, so `set -e` stops it where `script/projectname` fails.
 
     Inside a phase the tool is invoked directly — `golangci-lint`, `go test`,
     `eslint`, `prettier` — never through `make lint` or `script/test`, which are