Set UPAAS_PORT as well as PORT in docker-compose.yml (closes #230)

upaas reads UPAAS_PORT before PORT, so a UPAAS_PORT line in .env made it
listen on another port than the one the compose file's port mapping and
healthcheck use, and the container never became healthy. The compose file
now sets both to 8080, and its comment and the README's Docker Compose
section say that both are set.

Model: opus-5-5

docker-compose.yml

@@ -7,9 +7,11 @@ services:
     # Every line of .env is passed to upaas as an environment variable.
     env_file: .env
     environment:
-      # Overrides any PORT in .env, so upaas listens where the port mapping
-      # and healthcheck below expect it.
+      # Override any PORT or UPAAS_PORT in .env, so upaas listens where the
+      # port mapping and healthcheck below expect it. Both are set because
+      # upaas reads UPAAS_PORT first.
       PORT: "8080"
+      UPAAS_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