Make the image build multi-stage and cache-proof (closes #4)

The image build reported a green it had not earned. `script/cibuild` is a
bare `docker build .`, and with `COPY . .` followed by `RUN make check`,
an unchanged tree served that layer from cache: the suite never ran and
the build still exited 0, while the script's header comment asserted the
opposite.

CHECK_EPOCH, passed by `script/cibuild` and `script/docker`, changes the
cache key of the check and build layers on every invocation. It is
guarded, because an unset ARG is the empty string and therefore a stable
key: without the guard a plain `docker build .` — the command the policy
names, and the one anyone debugging types — would still get the false
green. A missing argument is now a hard failure rather than a silent
degradation to the behaviour the epoch was added to prevent.

The Dockerfile is now two stages: `fmt-check` and `lint` run first, and
the check stage takes a `COPY --from=lint` dependency on them, so a
formatting mistake fails the build in seconds instead of racing the
suite to the finish. Both stages stay pinned to the same digest.

The remaining fixes are one-liners that had made the target unusable:
`script/projectname` still printed the pre-rename name, so `make docker`
tagged its image after a name this project dropped in May;
`script/bootstrap` installed without fetching apt's package lists, which
cannot work on a Debian base; and `.dockerignore` had drifted far enough
from `.gitignore` to ship a ~100 MB compiled binary and any agent
worktree under `.claude/` into the build context. The second of those is
a correctness problem, not a size one — vitest globs a copied worktree's
tests alongside the real ones and runs the suite twice over. `.gitignore`
itself stays in the context, because prettier reads it as a default
ignore file and dropping it would change what `make fmt-check` sees.

TODO.md

@@ -18,6 +18,16 @@ Update the README API reference section to match the current implementation.
 
 # Completed Steps
 
+- 2026-08-09: Made `make docker` green and policy-conformant. Multi-stage
+  Dockerfile: a lint stage runs `make fmt-check` and `make lint`, and the check
+  stage takes a `COPY --from=lint` dependency on it before running `make check`
+  and `make build`. `CHECK_EPOCH` and a fail-closed guard stop Docker serving
+  those two layers from cache, which is what let a build report success without
+  running the suite. `script/projectname` says `quak`, so the image is tagged
+  `quak`; `script/bootstrap` updates apt lists before installing, so a Debian
+  base works; `.dockerignore` no longer ships the compiled binary, the caches or
+  agent worktrees into the build context, and keeps `.gitignore` in it for
+  prettier.
 - 2026-08-09: Fixed the TypeScript build. `rootDir` is the repo root, so `bin/`
   compiles alongside `src/` instead of failing with TS6059; output is
   `dist/src/` and `dist/bin/`, which is where `main`, `types` and `bin.quak` now
@@ -56,7 +66,6 @@ Update the README API reference section to match the current implementation.
 
 # Future Steps
 
-- Make `make docker` green.
 - Tag v1.0.0.
 - Future desktop client, separate repo:
     - Electron app skeleton consuming this library.