Boost composer-asset-plugin update speed

Comparing #3 with #4

Content

[...]
volumes:
[...]
- ~/.composer/cache:/root/.composer/cache

Example [Docker image with pre-installed plugins & Yii 2.0 runtime](https://github.com/dmstr/docker-php-yii2)

 
 
## Using asset-packagist
 
 
[Asset-packagist](https://asset-packagist.org/) is a proxy website which exposes npm and bower packages as composer packages. It works way faster and doesn't require any plugins.