suggest  support customize command to execute or support docker save -o ***.tar.gz **  to save image to dist

suggest  support customize command to execute
such as:

```yaml
setting:
  commands:
    - docker save -o ***.tar.gz ** 
    - docker tag **
```
or support docker save -o ***.tar.gz ** in settings plugin args

 i want to save a image tat.gz file to drone/src, thanks !