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, and
UPAAS_PLAINTEXT_HTTP is left unset. A healthcheck against /health uses
the runtime image's busybox wget.

The README's plain-HTTP Compose example becomes a short deploy section
that points at the file. .env is added to .dockerignore so settings are
not copied into the build.

Model: opus-5-5

docker-compose.yml

@@ -0,0 +1,24 @@
+# Runs upaas. Put settings in .env next to this file; see "Deploying with
+# Docker Compose" in README.md.
+services:
+  upaas:
+    build: .
+    restart: unless-stopped
+    # Every line of .env is passed to upaas as an environment variable.
+    env_file: .env
+    environment:
+      # 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}
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock
+      - ${HOST_DATA_DIR:?set HOST_DATA_DIR in .env to an absolute host path}:/var/lib/upaas
+    # Loopback only, for a TLS-terminating reverse proxy in front. Leave
+    # UPAAS_PLAINTEXT_HTTP unset behind that proxy.
+    ports:
+      - "127.0.0.1:8080:8080"
+    healthcheck:
+      test: ["CMD", "wget", "-q", "-O", "/dev/null", "http://127.0.0.1:8080/health"]
+      interval: 30s
+      timeout: 5s
+      retries: 3