Stop script/lint writing an image it never uses (closes #48)

script/lint builds Dockerfile.lint only for the exit status, but every
run exported the result as an image: seconds spent exporting, and one
untagged image left behind each time. It now builds with
--output=type=cacheonly, so nothing is exported. CHECK_EPOCH still
changes on every run, so the gate steps still run each time; the build
cache is kept as before.

Model: opus-5-5

script/lint

@@ -14,6 +14,10 @@
 # 0 in well under a second having run no linter. The PID is in the value
 # as well as the epoch because two lint runs land inside the same second
 # easily, and `date +%s` alone would cache the second one.
+#
+# The result is the build's exit status and the image is never used, so
+# --output=type=cacheonly writes none. Without it every run spends
+# seconds exporting an image and leaves it behind untagged.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -22,6 +26,7 @@ main() {
     cd "$ROOT"
     docker build \
         --build-arg CHECK_EPOCH="$(date +%s)-$$" \
+        --output=type=cacheonly \
         -f Dockerfile.lint \
         .
 }