Lint and test as phases of the Dockerfile (closes #38)

Linting and testing are now the lint and test phases of the one
Dockerfile, and the build stage copies a file from each, so a plain
docker build . cannot pass while either fails. Dockerfile.lint is gone.
REPO_POLICIES.md, script/lint, script/test, script/docker and
script/cibuild are the current copies from the next branch of
sneak/prompts: every docker build there is uncached and tagged,
script/test builds the test phase instead of running go vet and go test
on the host, and script/cibuild bootstraps, runs script/check, then
builds the image with the version from the host. The test phase
installs gcc and musl-dev because -race needs cgo. The version stamping
is unchanged. The issue's line leaves the README TODO list.

Model: opus-5-5

script/test

@@ -1,13 +1,19 @@
 #!/bin/sh
-# script/test: run the test suite (vet first, verbose rerun on failure).
+# 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 "$ROOT"
-    go vet ./...
-    go test -timeout 90s ./... || go test -timeout 90s -v ./...
+    docker build --no-cache \
+        --target test \
+        -t "$("$SCRIPT_DIR/projectname")-test" .
 }
 
 main "$@"