Suggestion on volume mount and env variables for custom image test command.

### Expected behavior

```yml
...
artifacts:
- image: service
- image: cukes
docker:
dockerfile: Dockerfile.test

test:
- image: cukes
custom:
- command: "60"
timeoutSeconds: 60
dependencies:
paths:
- features

```

The docker compose command

```
version: '2.1'
services:
service-tests:
environment:
- SERVICE_ENDPOINT=http://service:8089
- SPIN=${SPIN:-false}
volumes:
- "./test-results:/cucumber/test-output"
build:
context: .
dockerfile: Dockerfile.test
command: "60"
depends_on:
- service
```

### Actual behavior

### Information

- Skaffold version: : v1.39.2
- Operating system:
- Installed via: Homebrew
- Contents of skaffold.yaml - above

### Steps to reproduce the behavior

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