Add a Docker HEALTHCHECK and make docker-smoke (closes #111)

The runtime stage declares a HEALTHCHECK that probes
/.well-known/healthcheck.json with busybox wget. script/docker-smoke
(make docker-smoke) builds the image with script/docker, starts it with
a random PIXA_SIGNING_KEY, and passes only once Docker reports the
container healthy within 30 seconds; the container is removed on exit
and its log printed on failure. The Gitea workflow runs it after
script/cibuild; it is not part of make check.

It waits on Docker's health status instead of polling a published host
port because the Gitea job runs in its own container on its own
network, where such a port is not reachable at localhost.

Model: opus-5-5

README.md

@@ -176,6 +176,7 @@ them. We provide:
 - `script/fmt-check` — check formatting (read-only)
 - `script/check` — run test, lint, and fmt-check
 - `script/docker` — build the Docker image tagged via `script/projectname`
+- `script/docker-smoke` — build the image, start it, wait for it to be healthy
 - `script/cibuild` — CI entrypoint: `docker build .` (the Dockerfile
   runs the checks, so a green build implies a green repo)
 - `script/precommit` — pre-commit checks (`go mod tidy` guard, then