Improve distinction between builder and build-environments

### Expected behavior

### Actual behavior
WORST DOCUMENTATION EVER, AND NO WAY TO TELL WHAT IS WHAT AND WHERE ... HORRIFIC EXPERIENCE, WRITTEN BY FOLKS WHO HAVE NO IDEA HOW TO WRITE ANY DOCO WHATSOEVER

To use the 'docker' builder, remove the 'cluster' stanza from the 'build' section of your configuration.

But your doco shows they go hand in hand, and yes the api version matches .

COMPLETE WASTE OF TIME
### Information

- Skaffold version:
- Operating system:
- Installed via:
- Contents of skaffold.yaml:

```yaml

```

### Steps to reproduce the behavior

1. a clonable repository with the sample skaffold project
2. `skaffold `
3. ...