Add a warning/ error if there is whitespace around the '=' in environment variables

### Description

When environment variables using the mapping syntax are used in compose, whitespace around the equals character causes issues, so it would be nice to have some kind of warning, or an error if it is detected.

Example:
```
services: 
  web:
    build: .
    ports:
      - "8000:5000"
    environment:
      - DEBUG=true   # Correct
      - DEBUG = true # Causes issues
```