Run the tests under the race detector, in Docker (closes #23)

script/test is now the standard script: it builds only the test stage
of the Dockerfile, without the build cache, so tests no longer run on
the host. That stage calls go test -race -cover directly, with a
verbose rerun on failure, on the Debian-based golang image, which has
the C toolchain -race needs.

The test stage no longer copies from the lint stage, so script/test
builds the tests alone. A new final stage copies one file from each of
the two stages; that is what makes a plain docker build, and so
script/cibuild, run both. The README entry for script/test and TODO.md
say what now runs.

Model: opus-5-5

README.md

@@ -145,7 +145,9 @@ alpine. We provide:
   `script/bootstrap`, then `script/install-precommit`
 - `script/projectname` — output the project name (our own extension); used by
   `script/docker` for the image tag
-- `script/test` — run the test suite (`go test -v ./...`)
+- `script/test` — run the test suite under the race detector in Docker by
+  building only the `test` stage of the `Dockerfile`, without the build cache,
+  so every run tests; the image is tagged `simplelog-test`
 - `script/lint` — run golangci-lint in Docker by building only the `lint`
   stage of the `Dockerfile` (which also runs the format check), without the
   build cache, so every run lints; the image is tagged `simplelog-lint`