Check builds keep Go's modules and caches between runs (refs #118)

On sneak's rulings in sneak/prompts#113 and
sneak/prompts#130. REPO_POLICIES.md and
script/test, script/lint, script/docker and script/cibuild are the
sneak/prompts copies at 0653579: the scripts pass a new CACHEBUST value
instead of --no-cache, and the lint and test builds write no image. In
each Dockerfile stage the system packages and the Go module download
come above ARG CACHEBUST; every step that lints, tests or compiles, and
the node stages' yarn install, comes after it. Go's build cache and
golangci-lint's are cache mounts, and both go test runs carry -count=1.
README.md and TODO.md say the same.

Model: opus-5-5

script/docker

@@ -1,8 +1,12 @@
 #!/bin/sh
 # script/docker: build the Docker image tagged with the project name.
 # Identical in all repos; the tag comes from script/projectname.
-# --no-cache because the gate phases the final stage depends on are RUN
-# steps, and a cached one is a check that did not run.
+# A new CACHEBUST value reruns every RUN step after the ARG CACHEBUST
+# line, since a cached gate phase is a check that did not run; unlike
+# --no-cache, it keeps the layers above that line and the cache mounts.
+# This relies on every stage of the Dockerfile declaring ARG CACHEBUST
+# before every step that lints, tests or compiles; a stage without that
+# line is served from cache on an unchanged tree, with no warning.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -10,15 +14,17 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    # Own line: a failing command substitution inside an argument does
-    # not trip `set -e`, so the inline form degrades silently to an
-    # empty constant. The VERSION build argument takes precedence over
-    # the version a build stage derives from the .git in the context.
+    # The version and the tag each get their own line: a failing
+    # command substitution inside an argument does not trip `set -e`,
+    # so the inline form degrades silently to an empty constant. The
+    # VERSION build argument takes precedence over the version a build
+    # stage derives from the .git in the context.
     version="$(git describe --tags --always --dirty 2>/dev/null || true)"
     [ -n "$version" ] || version="unknown"
-    docker build --no-cache \
+    tag="$("$SCRIPT_DIR/projectname")"
+    docker build --build-arg CACHEBUST="$(date +%s)-$$" \
         --build-arg VERSION="$version" \
-        -t "$("$SCRIPT_DIR/projectname")" .
+        -t "$tag" .
 }
 
 main "$@"