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

Dockerfile

@@ -29,9 +29,9 @@ WORKDIR /app
 COPY --from=builder /src/bin/upaasd /app/upaasd
 
 # Create data directory
-RUN mkdir -p /data
+RUN mkdir -p /var/lib/upaas
 
-ENV UPAAS_DATA_DIR=/data
+ENV UPAAS_DATA_DIR=/var/lib/upaas
 ENV UPAAS_PORT=8080
 
 EXPOSE 8080