Gate the build on Docker lint and test phases (closes #40, closes #30)

Per the owner ruling on issue 40, linting and testing are phases of the
main Dockerfile rather than a separate lint file. script/lint and
script/test build one phase each by name with caching disabled, and the
final stage copies a harmless file from each so the image cannot be built
unless both passed. A stage that is not the last is built only when
something depends on it or --target names it, so the gates are invoked by
name and the edges kept. script/check runs the gates and builds no image
of its own; script/cibuild bootstraps first, because CI runs it alone and
fmt-check is native. fmt and fmt-check source nvm for the pinned node
before calling yarn, which bootstrap installs but leaves off its caller's
PATH. Every build in script/ is tagged and uncached. Issue 30 closes too:
a container has its own lint cache and lock.

Model: opus-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 "$ROOT"
-    echo "No tests defined."
+    docker build --no-cache \
+        --target test \
+        -t "$("$SCRIPT_DIR/projectname")-test" .
 }
 
 main "$@"