Run every lint-class check inside Docker (closes #38)

Add a root Dockerfile.lint that carries the checks as build steps -- a
`lint` stage running `hugo --minify --printPathWarnings` and a
`fmt-check` stage running the prettier check -- and reduce script/lint
and script/fmt-check to building their stage. A successful build is a
clean check. There is no host path and deliberately no "am I already
inside a container?" branch, which would be a host lint path in
disguise.

The two stages share a `base` whose first four instructions are
byte-identical to the main Dockerfile's, so the expensive
`RUN script/bootstrap` layer that compiles the pinned Hugo from source
is a cache hit against the main image instead of a second build of the
same thing.

Resolve the resulting recursion by splitting the checks by where they
run, not with an escape hatch. `make check` runs script/lint, so the
main Dockerfile can no longer `RUN make check`: that would be
docker-in-docker inside a bare Alpine with no docker client and no
daemon socket, and script/cibuild is what CI runs on every push. The
main Dockerfile therefore runs `make test`, the production build, and
script/cibuild builds it and then calls script/lint and
script/fmt-check. CI still covers the production build, lint and the
format check, and it runs exactly what a developer runs.

script/fmt stays on the host because it rewrites the working tree,
which a container build cannot do. That makes it the authoritative
copy of the prettier version, scope and flags that the fmt-check stage
duplicates; both sides carry a keep-in-sync note. The duplication is
forced: any `RUN script/fmt-check` inside the image is the recursion
again.

Caching is waived for the checks in the shape this repo already
settled: `ARG CHECK_EPOCH` with no default, declared and guarded
separately in each stage because ARG does not cross a FROM, with the
value expanded into the checked command as well as the guard so
invalidation does not rest on BuildKit's treatment of an unreferenced
ARG. All four image-building entrypoints now generate and pass it --
script/cibuild, script/docker, script/lint, script/fmt-check.

Verified: two consecutive script/lint runs on an unchanged tree both
executed hugo for real, with script/bootstrap CACHED; a constant-epoch
counterfactual restored the false green (exit 0, lint layer CACHED, no
hugo output); an empty epoch failed closed on the guard; a broken
template failed the lint stage and an unformatted README failed the
fmt-check stage, both reverted and re-run clean; script/cibuild and
`make check` are green with all three checks demonstrably executing.

script/lint

@@ -1,15 +1,26 @@
 #!/bin/sh
-# script/lint: this Hugo site has no dedicated linter, so the lint gate
-# is a clean build that surfaces broken internal links and template
-# path problems. It is a real check: `hugo` fails on build errors, and
-# --printPathWarnings reports render-target collisions.
+# script/lint: run the lint. This Hugo site has no dedicated linter, so
+# the lint gate is a clean build that surfaces broken internal links and
+# template path problems -- but where it runs is not negotiable: every
+# lint run happens inside a Docker container, so this script does
+# nothing except build the `lint` stage of Dockerfile.lint. The check is
+# a build step there, so a successful build is a clean lint. There is
+# deliberately no host fallback and no "already inside a container?"
+# branch: either would be a host lint path wearing a disguise.
+#
+# Dockerfile.lint requires the CHECK_EPOCH build argument, generated
+# here exactly as script/cibuild generates it -- see that script for why
+# the check layer must not be allowed to cache, and why the value is
+# built in an assignment rather than inline.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    hugo --minify --printPathWarnings
+    epoch="$(date +%s%N)$$"
+    docker build -f Dockerfile.lint --target lint \
+        --build-arg CHECK_EPOCH="$epoch" .
 }
 
 main "$@"