The base image never starts a service (closes #16)

build.sh and bin/test used to start valkey-server and ping it to prove it runs;
they now check it with valkey-server --version and valkey-cli --version, so the
ping loop, shutdown, and the cleanup of the Valkey run's log files are gone.
/var/log/valkey and /var/log/nginx keep their package owners and modes, and
nginx -t stays. README states that Valkey, PostgreSQL and nginx are installed
but never run. No package install starts a service: invoke-rc.d already denies
every start, and no ENTRYPOINT or CMD starts one.

Model: deepseek-v4-pro

README.md

@@ -140,6 +140,9 @@ exec /usr/local/bin/myapp
 Services should log to stdout/stderr; the container's log is the combined
 output of all services.
 
+The image installs Valkey, PostgreSQL and nginx but never runs them; a child
+image that wants one runs it as a service.
+
 The image sets `CMD ["/usr/local/sbin/runsvinit"]` and no `ENTRYPOINT`, so
 a child image that only needs one unsupervised process can override `CMD`
 and ignore runit entirely, and `docker run -it image bash` works as