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, the tests run as nobody so the
tests that make a file unreadable still see the read fail, and the
container is removed afterwards. It is not part of make check. The
detector found no races.

Model: opus-5-5

TODO.md

@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- `make test-race` runs the test suite under the race detector in a
+  cgo-enabled container, outside `make check` (2026-10-04,
+  https://git.eeqj.de/sneak/sfdupes/issues/18)
+
 - tests cover a missing database, `scan` keeping stdout empty, its skip
   warning, the `report` and `trees` summary lines, and every subcommand
   going through `runE` (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/16)
@@ -538,5 +542,7 @@ Accepted divergences (no action):
 - flat single-package layout with `.go` files in the repo root — fine
   for a small single-binary tool per the Go styleguide; the tracker
   audit agrees
-- `go test` runs without `-race` — the repo mandates `CGO_ENABLED=0`
-  (pure-Go builds) and the race detector requires cgo
+- `make test` runs without `-race` — the repo mandates `CGO_ENABLED=0`
+  (pure-Go builds) and the race detector requires cgo, so the detector
+  runs in a separate cgo-enabled container, `make test-race`, which is
+  not part of `make check`