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/docker

@@ -1,8 +1,8 @@
 #!/bin/sh
 # script/docker: build the Docker image tagged with the project name.
 # Identical in all repos; the tag comes from script/projectname.
-# --no-cache for the same reason as script/cibuild: a cached check layer
-# is a check that did not run.
+# --no-cache because the gate phases the final stage depends on are RUN
+# steps, and a cached one is a check that did not run.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"