Fix: Resolve permission issues with .storage directories

- Install su-exec in Dockerfile for user switching
- Modified start.sh to create directories as root, then change ownership
- Container starts as root but switches to nextjs user for app execution
- This prevents permission denied errors when creating .storage directories

start.sh

@@ -1,11 +1,14 @@
 #!/bin/sh
 
-# Create .storage directories if they don't exist
+# Create .storage directories if they don't exist (as root)
 mkdir -p /app/.storage/users
 mkdir -p /app/.storage/bookings
 mkdir -p /app/.storage/treatments
 mkdir -p /app/.storage/availability
 mkdir -p /app/.storage/cancellation-tokens
 
-# Start the application
-exec node server-dist/index.js
+# Change ownership to nextjs user
+chown -R nextjs:nodejs /app/.storage
+
+# Start the application as nextjs user
+exec su-exec nextjs node server-dist/index.js