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

Dockerfile

@@ -76,4 +76,7 @@ WORKDIR /var/lib/pixa
 
 EXPOSE 8080
 
+HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
+    CMD wget --spider -q http://localhost:8080/.well-known/healthcheck.json || exit 1
+
 ENTRYPOINT ["/usr/local/bin/pixad", "--config", "/etc/pixa/config.yml"]