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

TODO.md

@@ -30,6 +30,12 @@ exhaustion
 
 # Completed Steps
 
+- 2026-09-28 Docker image healthcheck (closes #111): a `HEALTHCHECK` in
+  the runtime stage probing `/.well-known/healthcheck.json` with busybox
+  `wget`; `script/docker-smoke` (`make docker-smoke`) builds the image,
+  starts it with a throwaway `PIXA_SIGNING_KEY`, and passes only once
+  Docker reports it healthy within 30 seconds, removing the container on
+  exit; the Gitea workflow runs it after `script/cibuild`.
 - 2026-09-21 trusted-proxy client IP resolution (closes #94): a
   `trusted_proxies` config key taking a list of CIDRs, parsed by the same
   `net/netip` list parser as `blocked_networks` (an invalid entry aborts