build: streamline Docker setup and environment config

- Simplify Dockerfile to use official Bun image as base
- Add admin bootstrap environment variables to .env.example
- Include VITE_ADMIN_EMAIL and VITE_ADMIN_PASSWORD build args
- Fix newline at end of .env.example file
- Remove redundant system dependency installations

.env.example

@@ -20,6 +20,12 @@ VITE_COUCHDB_URL=http://localhost:5984
 VITE_COUCHDB_USER=admin
 VITE_COUCHDB_PASSWORD=change-this-secure-password
 
+# Default Admin Bootstrap (used by frontend seeder at startup)
+# Note: These are evaluated at build-time by Vite. If you change them,
+# rebuild the frontend image (`docker compose build frontend`).
+VITE_ADMIN_EMAIL=admin@localhost
+VITE_ADMIN_PASSWORD=admin123!
+
 # Application Configuration
 # Base URL for your application (used in email links)
 # Development: http://localhost:5173
@@ -85,4 +91,4 @@ GITEA_REPOSITORY=yourusername/rxminder
 DEPLOYMENT_WEBHOOK_URL=
 
 # Image cleanup settings
-CLEANUP_OLD_IMAGES=true
+CLEANUP_OLD_IMAGES=true