Docker Compose Multi-Service Setup

**Context**: 
Docker Compose is a tool for defining and running multi-container Docker applications. It allows users to define a configuration file, typically named `docker-compose.yml`, which describes the services, networks, and volumes that make up the application. In a multi-service setup, multiple containers are launched and managed together, enabling complex applications to be deployed and managed as a single entity.

**Detailed Instructions**: 
Create a Docker Compose configuration for a multi-service application consisting of a web server, a database, and a caching layer. The web server should use the latest version of the official Python image and expose port 80. The database should use the latest version of PostgreSQL and persist data in a named volume. The caching layer should use Redis and expose port 6379. Ensure that all services can communicate with each other over a custom network named "app-net". Define environment variables for the database connection in the web server service. 

The configuration should include:
- Service definitions for the web server, database, and caching layer.
- A custom network named "app-net" for service communication.
- A named volume for persisting database data.
- Environment variables for database connection settings in the web server service.
- Port mappings for exposing the web server and caching layer services.

**Output Format**: 
The output should be a complete `docker-compose.yml` file that can be used to create and start the multi-service application. The file should be formatted in YAML and include all necessary configuration options as specified in the instructions.

**Examples**: 
For reference, a simple service definition in Docker Compose might look like:
```yml
version: '3'
services:
  web:
    image: python:latest
    ports:
      - "80:80"
```
However, the final output should include all services (web server, database, caching layer), the custom network, named volume, environment variables, and port mappings as detailed in the instructions.