build: run all linting in Docker via Dockerfile.lint (closes #134)

golangci-lint is no longer installed or run on the host. script/lint is
now a thin wrapper that builds the new root Dockerfile.lint, which COPYs
the repo into the digest-pinned golangci/golangci-lint:v2.12.2 image and
lints as a build step, so a successful build is a clean lint. This works
even where the docker daemon is remote and bind mounts are impossible.

Dockerfile.lint is split into a deps stage (base image, go mod download)
and a lint stage (source copy, linter run). script/lint passes
--no-cache-filter=lint so the lint stage executes on every invocation:
caching is explicitly waived for linting, and a cached build lints
nothing. The deps stage stays cached and no global cache invalidation is
performed. --progress=plain keeps the linter's own output visible.

golangci-lint config verify is deliberately omitted: it fetches its JSON
schema over a live, unpinned HTTPS call, which would make linting
network-dependent and defeat hash-pinning.

script/bootstrap no longer installs golangci-lint and warns instead when
docker is absent. The goimports install stays, since script/fmt and
script/fmt-check still run it on the host.

The root Dockerfile ran make check in its builder stage, which would now
recurse into script/lint and shell out to docker build with no daemon
available. It gains its own lint stage on the same pinned image, invoked
directly, with the builder depending on it via COPY --from=lint and
running make fmt-check, make test and make build.

TODO.md

@@ -25,6 +25,16 @@ confirm make check still passes.
 
 # Completed Steps
 
+- 2026-08-10: all linting moved into Docker: new root `Dockerfile.lint`
+  on the digest-pinned `golangci/golangci-lint:v2.12.2` image,
+  `script/lint` reduced to a thin wrapper that builds it with
+  `--no-cache-filter=lint` so the linter actually executes every run,
+  golangci-lint install dropped from `script/bootstrap` (goimports
+  stays, `script/fmt` needs it on the host), and the root `Dockerfile`
+  given its own lint stage so its build no longer recurses through
+  `make check` into `script/lint`. `golangci-lint config verify` is
+  deliberately omitted: it fetches its schema over an unpinned live
+  HTTPS call
 - 2026-08-07: golangci-lint bumped to v2.12.2 (commit-pinned installs
   in `Dockerfile` and `script/bootstrap`); `.golangci.yml` set to the
   org-standard v2-schema config used across the org's repos