Auto-generate and persist session secret on first startup

- Generate random 32-byte session secret if not set via env var
- Persist to $UPAAS_DATA_DIR/session.key for container restarts
- Load existing secret from file on subsequent startups
- Change container data directory to /var/lib/upaas

README.md

@@ -170,10 +170,12 @@ Environment variables:
 docker run -d \
   -p 8080:8080 \
   -v /var/run/docker.sock:/var/run/docker.sock \
-  -v upaas-data:/data \
+  -v upaas-data:/var/lib/upaas \
   upaas
 ```
 
+Session secrets are automatically generated on first startup and persisted to `$UPAAS_DATA_DIR/session.key`.
+
 ## License
 
 MIT