Add docker-compose.yml for deploying upaas (closes #223)

The compose file builds the image from this repo, mounts the Docker
socket and HOST_DATA_DIR (passed to upaas as UPAAS_HOST_DATA_DIR),
reads settings from .env, and restarts unless stopped. The port is
published on 127.0.0.1 only, for a TLS-terminating proxy in front.
PORT and UPAAS_DATA_DIR are pinned so .env cannot move upaas off the
port mapping, the healthcheck (busybox wget) or the data mount.

upaas now refuses to start when UPAAS_HOST_DATA_DIR is set to a
relative path; when unset it still falls back to the data directory.

The README's plain-HTTP Compose example becomes a short deploy section
that points at the file. .env is added to .dockerignore.

Model: opus-5-5

README.md

@@ -230,8 +230,9 @@ HOST_DATA_DIR=/srv/upaas/data
 ```
 
 Other settings from [Configuration](#configuration) go in the same file, except
-`PORT`: the compose file sets it to 8080, overriding `.env`, to match its port
-mapping and healthcheck. Then run `docker compose up -d` from the repo root;
+`PORT` and `UPAAS_DATA_DIR`: the compose file sets them to 8080 and
+`/var/lib/upaas`, overriding `.env`, to match its port mapping, healthcheck and
+data directory mount. Then run `docker compose up -d` from the repo root;
 `docker compose ps` shows the container as healthy once `/health` answers.
 
 **Important**: `HOST_DATA_DIR` **must** be an **absolute path** on the host. It

docker-compose.yml

@@ -10,6 +10,9 @@ services:
       # Overrides any PORT in .env, so upaas listens where the port mapping
       # and healthcheck below expect it.
       PORT: "8080"
+      # Overrides any UPAAS_DATA_DIR in .env, so the database stays on the
+      # HOST_DATA_DIR mount below instead of inside the container.
+      UPAAS_DATA_DIR: /var/lib/upaas
       # The Docker daemon resolves app bind mounts on the host, so upaas must
       # know the host path of its data directory.
       UPAAS_HOST_DATA_DIR: ${HOST_DATA_DIR:?set HOST_DATA_DIR in .env to an absolute host path}