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

TODO.md

@@ -29,6 +29,9 @@
 
 # Completed Steps
 
+- `script/lint` writes no image, so a run no longer leaves an untagged
+  one behind (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/48)
+
 - `.golangci.yml` replaced with the current canonical copy, which uses
   `gomodguard_v2`, so lint no longer prints a deprecation warning
   (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/26)