Run the tests under the race detector with make test-race (closes #18)

script/test-race runs go test -race in a digest-pinned Debian golang
image that has gcc, since the detector needs cgo and the build keeps it
off. The checkout is mounted read-only and the container is removed
afterwards. The tests run as the calling user, or as nobody when that is
root, so the tests that make a file unreadable still see the read fail.
It is not part of make check. The detector found no races.

Model: opus-5-5

README.md

@@ -742,14 +742,23 @@ entrypoints are:
   installed: they run in Docker (see `script/lint` and `script/fmt`) and never
   from a host install, so there is no host copy to drift from the pin. A missing
   `docker` is warned about rather than installed or treated as fatal —
-  everything except linting and formatting works without it. Ends with
-  `go mod download`.
+  everything except linting, formatting and `make test-race` works without it.
+  Ends with `go mod download`.
 - `script/setup` — make a fresh clone ready for development: runs
   `script/bootstrap`, then `script/install-precommit`.
 - `script/projectname` — print this project's name (`sfdupes`). Scripts that
   need the name call it, so they stay identical across repositories.
 - `script/test` — run the test suite with a 30-second timeout and coverage
   enabled, rerunning verbosely on failure so the logs show which test failed.
+- `script/test-race` — run the test suite under the race detector with a
+  60-second timeout. The detector needs cgo and a C compiler, which the build
+  never uses, so the tests run in a digest-pinned Debian `golang` image that has
+  `gcc`, with the checkout mounted read-only; the container is removed when it
+  exits. They run as the calling user, or as `nobody` when that is root, because
+  several tests make a file unreadable and root reads it anyway; only then must
+  the checkout be readable by other users. Not part of `script/check`. Every run
+  starts with empty caches, so it needs the network and takes minutes, and the
+  mount needs a local docker daemon.
 - `script/lint` — run the linter. It builds `Dockerfile.lint`, which copies the
   repository into the digest-pinned `golangci/golangci-lint` image and runs
   `golangci-lint config verify` and `golangci-lint run` as build steps, so a
@@ -840,6 +849,8 @@ compile recipe:
 - `make setup` — prepare a fresh clone: `bootstrap` plus the pre-commit hook.
 - `make test` — run the test suite (30-second timeout; reruns with `-v` on
   failure).
+- `make test-race` — run the test suite under the race detector, in Docker (see
+  `script/test-race`); requires `docker`. Not part of `make check`.
 - `make lint` — run `golangci-lint` with the repo config, in Docker (see
   `script/lint`); requires `docker`.
 - `make fmt` / `make fmt-check` — format the Go sources and the Markdown /