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

.dockerignore

@@ -1,4 +1,5 @@
 .git
+.env
 bin/
 .editorconfig
 .vscode/