Start on a fresh upaas volume and document running under upaas (closes #129)

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

TODO.md

@@ -30,6 +30,12 @@ exhaustion
 
 # Completed Steps
 
+- 2026-09-28 start on a fresh upaas volume (closes #129): the image
+  starts as root only to give `/var/lib/pixa` to `pixad` when `pixad`
+  does not own it (`deploy/docker-entrypoint.sh`), then runs the server
+  as `pixad` through `su-exec`, so a root-owned host directory
+  bind-mounted there no longer stops the container at startup;
+  `README.md` gains a "Running under upaas" section.
 - 2026-09-28 run all linting in Docker via `Dockerfile.lint` +
   `script/lint` (closes #104): `make lint` calls `script/lint`, the only
   way the linter is run; inside a container (both Dockerfiles set