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 each build one phase by name with caching disabled, and the
final stage copies a harmless file from each so the image cannot be built
unless both passed — the ordering trick already in template-app-go's
Dockerfile, extended to the test phase. A phase that is not the last
stage is built only when something depends on it or --target names it, so
the gates are always invoked by name and the edges are kept. Every build
in script/ is tagged. No config verify step; fmt stays on the host. This
closes issue 30 too: a container has its own result cache and lock.

Model: opus-5

Dockerfile

@@ -1,15 +1,58 @@
+# Lint phase. The linter is invoked directly rather than through `make
+# lint` or `script/lint`, which are themselves a docker build and would
+# recurse into a daemon that does not exist in a build step.
+#
 # node 22-alpine, 2026-02-22
-FROM node@sha256:e4bf2a82ad0a4037d28035ae71529873c069b13eb0455466ae0bc13363826e34
+FROM node@sha256:e4bf2a82ad0a4037d28035ae71529873c069b13eb0455466ae0bc13363826e34 AS lint
 
 WORKDIR /app
 
-# script/bootstrap installs all prerequisites (make via apk here; node
-# and yarn are already in the base image, so those steps are skipped).
-# Dependency manifests are copied first so the bootstrap layer is
-# cached until they change.
 COPY script/ script/
 COPY package.json yarn.lock ./
 RUN script/bootstrap
 
 COPY . .
-RUN make check
+
+RUN yarn run prettier --check '**/*.md' --tab-width 4 --prose-wrap always
+
+# Test phase, same shape and for the same reason.
+#
+# node 22-alpine, 2026-02-22
+FROM node@sha256:e4bf2a82ad0a4037d28035ae71529873c069b13eb0455466ae0bc13363826e34 AS test
+
+WORKDIR /app
+
+COPY script/ script/
+COPY package.json yarn.lock ./
+RUN script/bootstrap
+
+COPY . .
+
+RUN echo "No tests defined."
+
+# Development environment, and the last stage: a plain `docker build .`
+# names no target and so builds this one. Nothing is wanted from the two
+# phases above; the copies are what make BuildKit build them first, so
+# this image cannot be produced unless lint and test passed. A stage
+# appended after this one would drop all three out of a plain build.
+#
+# node 22-alpine, 2026-02-22
+FROM node@sha256:e4bf2a82ad0a4037d28035ae71529873c069b13eb0455466ae0bc13363826e34
+
+WORKDIR /app
+
+COPY --from=lint /app/package.json /dev/null
+COPY --from=test /app/package.json /dev/null
+
+# script/bootstrap installs all prerequisites. Manifests are copied
+# first so that layer stays cached until dependencies change.
+COPY script/ script/
+COPY package.json yarn.lock ./
+RUN script/bootstrap
+
+COPY . .
+
+# The version is computed on the host and passed in, because
+# .dockerignore excludes .git.
+ARG VERSION=dev
+LABEL org.opencontainers.image.version="${VERSION}"