Add initial setup for pgAdmin 4 with Docker and Nginx Proxy Manager

- Created .env.example for environment variable configuration.
- Added .gitignore to exclude sensitive files.
- Introduced config_local.py for pgAdmin settings.
- Added docker-compose.portainer.yml for service orchestration.
- Created Dockerfile for custom image building.
- Added README.md with setup instructions and usage guidelines.
- Included servers.json as a template for PostgreSQL server connections.

Dockerfile

@@ -0,0 +1,4 @@
+FROM dpage/pgadmin4:latest
+
+COPY config_local.py /pgadmin4/config_local.py
+COPY servers.json /pgadmin4/servers.json