Run all linting in Docker through script/lint (closes #104)

make lint calls script/lint, the only way golangci-lint is run. Inside a
container it runs the linter; anywhere else it builds Dockerfile.lint,
whose last step runs script/lint again. Both Dockerfiles set
container=docker to mark the container, since /.dockerenv is missing in
build steps and present on hosts that are themselves containers. The
Dockerfile lint stage runs make lint.

A new CACHEBUST build-arg on every run keeps the lint step from being
served from cache; a tmpfs mount keeps Go's and golangci-lint's caches
out of that step's layer, so runs do not pile up build cache.
script/bootstrap and the nix-shell package lists no longer carry
golangci-lint. golangci-lint config verify is not run: it fetches its
schema over an unpinned live HTTPS call.

Model: opus-4-8 (implementation); opus-5-5 (rework)

Makefile

@@ -10,7 +10,7 @@ ifdef HAS_PKGCONFIG
   NIX_RUN_PREFIX =
   NIX_RUN_SUFFIX =
 else
-  NIX_RUN_PREFIX = nix-shell -p pkg-config vips libheif golangci-lint git --run '
+  NIX_RUN_PREFIX = nix-shell -p pkg-config vips libheif git --run '
   NIX_RUN_SUFFIX = '
 endif