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

script/test

@@ -1,12 +1,19 @@
 #!/bin/sh
-# script/test: run the test suite.
+# script/test: run the test suite. Testing is a phase of the Dockerfile
+# and this builds that phase alone, on the same terms as script/lint:
+# --target because a phase that is not the last stage is built only when
+# named, --no-cache because a cached test layer is a test that did not
+# run, and a tag so each build replaces the previous image.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    go test -v ./...
+    docker build --no-cache \
+        --target test \
+        -t "$("$SCRIPT_DIR/projectname")-test" .
 }
 
 main "$@"