Run the lint inside Docker via Dockerfile.lint (closes #38)

Add a root Dockerfile.lint that runs `hugo --minify --printPathWarnings`
as a build step, so a successful build IS a clean lint, and reduce
script/lint to building that file. There is no host lint path and
deliberately no "am I already inside a container?" branch, which would
be a host lint path in disguise.

The containerisation boundary is lint only, per the owner ruling on the
issue: formatting is not a lint, so script/fmt and script/fmt-check stay
on the host, unchanged in version, scope and flags. That also removes
the forced duplication of prettier's settings between a script and a
Dockerfile, and with it the keep-in-sync notes that duplication needed.

Dockerfile.lint has exactly one stage on purpose. A whole-file
`docker build -f Dockerfile.lint .` builds only the file's last stage,
and sibling stages off a shared base carry no ordering edge, so a second
stage beside the lint would be silently skipped by exactly the
invocation the canonical org-wide script/lint uses -- a green that
linted nothing, which the per-stage CHECK_EPOCH guard cannot catch
because the stage that did run satisfies it. With one stage there is
nothing to skip and script/lint needs no --target. A comment in the file
says that any second check added here must be chained or carry an
explicit ordering edge, never left as a sibling.

Its first four instructions are byte-identical to the main Dockerfile's
and in the same order, so the expensive `RUN script/bootstrap` layer
that compiles the pinned Hugo from source is shared between the two
images rather than paid twice.

Resolve the recursion by direction, not detection. `make check` calls
script/lint, and script/lint is now a `docker build`, so `RUN make
check` in an image would attempt a docker build inside a build step
where there is no daemon. The main Dockerfile therefore runs the
individual non-lint checks -- script/test and script/fmt-check, as
separate RUN lines under the CHECK_EPOCH guard -- matching the canonical
shape, and only the lint is absent from it. script/cibuild runs
script/lint first, for fail-fast feedback: on a runner with no cached
bootstrap layer a lint failure should not wait behind a Hugo build from
source. CI coverage is therefore unchanged, and it runs the same scripts
a developer runs.

Caching is waived for the lint in the shape this repo already settled:
ARG CHECK_EPOCH with no default, guarded with
`[ -n "$CHECK_EPOCH" ] || exit 1`, and the value expanded into the
linted command as well as the guard, so invalidation never rests on
BuildKit's treatment of an unreferenced ARG. Every image-building
entrypoint generates and passes it -- script/cibuild, script/docker,
script/lint -- each as a whole assignment rather than inline, for the
`set -e` reason script/cibuild documents.

script/lint builds with `--output type=cacheonly`: the build is run for
its exit status, not for an image, and because the lint layer is
cache-busted on every invocation an exporting build leaves one dangling
image per lint run. On a host shared with other work that accumulates.
The build cache is unaffected, so script/bootstrap still hits, and
failures still propagate.

Two divergences from REPO_POLICIES.md, stated rather than buried:

  - REPO_POLICIES.md:92, "all Dockerfiles must run `make check`". That
    rule and "every lint run happens in Docker" cannot both hold once
    `make check` contains the lint.
  - REPO_POLICIES.md:102-168, which requires a separate lint stage whose
    result the build stage depends on through
    `COPY --from=lint /src/go.sum /dev/null`, on the stated grounds that
    without the edge "the build stage would not wait for lint to finish
    and a lint failure might not fail the overall build". No such edge
    exists here: the lint is its own file and its own build, sequenced
    by script/cibuild rather than by BuildKit. Both sections are
    superseded upstream by 12e8db8 in sneak/prompts, which deletes the
    Go multistage lint stage and its ordering trick for the same reason
    -- that stage ran `make lint`, which is now a docker build.

Verified: two consecutive script/lint runs on an unchanged tree both
executed hugo for real, distinct epochs echoed, script/bootstrap CACHED,
second run 0.85s; a whole-file `docker build -f Dockerfile.lint .` with
the argument and no --target ran the lint for real; a bare build with no
argument failed closed on the guard; a planted template error failed the
lint with hugo's own render error and made script/cibuild exit non-zero
in 0.6s with the main image build never starting; a planted over-long
line failed the host script/fmt-check; both reverted and re-run clean;
`make check`, script/docker and script/cibuild all green with every
check layer observed executing rather than served from cache, and the
bootstrap layer CACHED in both images. The deploy path is byte-identical
to main: .gitea/, script/bootstrap, script/test and .dockerignore are
untouched.

README.md

@@ -39,6 +39,11 @@ build, and the formatting check:
 make check
 ```
 
+The lint runs inside Docker, so `make check` needs a working Docker daemon;
+there is no host fallback. On a machine that has never built the image, the
+first run compiles the pinned Hugo from source, which takes minutes; later runs
+reuse that cached layer.
+
 `make fmt` rewrites the repo's markdown and CSS to the project's prettier
 settings; run it if `make check` fails on formatting.
 
@@ -61,22 +66,37 @@ provide:
   git pre-commit hook
 - `script/test` — the correctness check: a clean `hugo --minify` production
   build
-- `script/lint` — a clean build that surfaces broken links and path collisions
+- `script/lint` — a clean build that surfaces broken links and path collisions,
+  run inside Docker: it builds `Dockerfile.lint`, where the lint is a build
+  step, so a successful build is a clean lint
 - `script/fmt` — format every markdown and CSS file in the repo with prettier;
   the exclusions live in `.prettierignore` with the reason for each
 - `script/fmt-check` — check that formatting (read-only)
 - `script/check` — run `script/test`, `script/lint`, then `script/fmt-check`;
   modifies no tracked files
 - `script/docker` — build the Docker image tagged with the project name
-- `script/cibuild` — the CI build; the Dockerfile runs `make check`
+- `script/cibuild` — the CI build: `script/lint` first, for fail-fast feedback,
+  then the main image, which runs the non-lint checks
 - `script/install-precommit` — install the git pre-commit hook that runs
   `script/check`
 
-Build the image through `script/cibuild` or `script/docker` only. Both pass a
-per-invocation `CHECK_EPOCH` build argument that the Dockerfile requires, so the
-`make check` layer can never be served from cache — without it Docker returns a
-green it did not earn. A bare `docker build .` fails closed on the Dockerfile's
-`CHECK_EPOCH` guard rather than caching its way to a false success.
+Every lint run for this repo happens inside a container, and only the lint does.
+`script/lint` has no host path and no "already inside a container?" branch, so
+what a developer runs and what CI runs are the same build. `script/fmt` and
+`script/fmt-check` run on the host: a formatting check is not a lint.
+
+That is also why the main `Dockerfile` runs `script/test` and `script/fmt-check`
+rather than `make check`. `make check` calls `script/lint`, which is itself a
+`docker build`, so a `make check` inside an image would attempt a docker build
+in a bare Alpine with no docker client and no daemon socket. The lint is not
+skipped — `script/cibuild` runs it first, in its own container, before the main
+image build starts.
+
+Build any image through `script/cibuild`, `script/docker` or `script/lint` only.
+All three pass a per-invocation `CHECK_EPOCH` build argument that the
+Dockerfiles require, so a check layer can never be served from cache — without
+it Docker returns a green it did not earn. A bare `docker build` fails closed on
+the `CHECK_EPOCH` guard rather than caching its way to a false success.
 
 A convenience `make serve` target runs `hugo server` for local preview.