Improvement in the Installation guide ( Docker Part ) #588

I tried to install coala via Docker using the following guide:
https://docs.coala.io/en/latest/Users/Install.html

Not being familiar with Docker, I would have saved some time if a link for the following page was in the install via Docker section:
http://docs.coala.io/en/latest/Users/Docker_Image.html

@li-boxuan Migrated the issue here (will start working as soon as I am assigned), thanks for helping!