upaas bind-mounts an existing host directory and sets no container
user, so a directory made with mkdir as root left pixad unable to
write /var/lib/pixa, and the container exited at startup.
The image now starts as root: deploy/docker-entrypoint.sh gives
/var/lib/pixa to pixad when pixad does not own it, then runs the
server as pixad through su-exec (alpine's package), so the server
never runs as root. README.md gains a "Running under upaas" section:
port, volume, environment variables, health check, first-run step.
Model: opus-5-5
