build: run all linting in Docker via Dockerfile.lint (closes #104)

golangci-lint now runs only inside a container, never on the host.
script/lint builds a hash-pinned root Dockerfile.lint; the nix-shell and
host golangci-lint paths are gone. A per-run CACHEBUST build-arg is
folded into the lint step's cache key, so the linter re-executes on every
run and an unchanged tree cannot return a cached success having linted
nothing; script/lint fails a build that did not run the linter.

Dockerfile's lint stage runs golangci-lint directly, since make lint now
builds a container and there is no Docker inside a build. It is the same
image and config. golangci-lint config verify is left out: it fetches its
schema over an unpinned live HTTPS call, which REPO_POLICIES.md forbids.

Model: opus-4-8

TODO.md

@@ -39,6 +39,17 @@ exhaustion
   (IPv4-mapped forms covered); enforcement stays in the dial-time
   re-resolution so the DNS-rebinding window remains closed; documented in
   `README.md` and `config.example.yml`.
+- 2026-09-21 run all linting in Docker via `Dockerfile.lint` +
+  `script/lint` (closes #104): `script/lint` builds a hash-pinned root
+  `Dockerfile.lint`, and no host or nix-shell `golangci-lint` path
+  remains (`script/bootstrap` installs no linter, and the Makefile and
+  `script/test` nix-shell package lists carry only build/test deps); a
+  per-run `CACHEBUST` build-arg forces the lint step to execute every
+  run, so an unchanged tree cannot return a cached green that linted
+  nothing; `Dockerfile`'s lint stage runs `golangci-lint` directly,
+  since `make lint` now builds a container and there is no Docker inside
+  a build; `golangci-lint config verify` stays out, as it fetches its
+  schema over an unpinned live HTTPS call
 - 2026-09-21 http.Server hardening (closes #92): added
   `HTTPReadHeaderTimeout` (10s, bounds the slowloris header dribble) and
   `HTTPIdleTimeout` (120s, bounds keep-alive reuse) alongside the