I strongly second this and can't overestimate how helpful this would be or how often I would use it.

I Have the Solution

But I really don't like the text editor for this forum so I created a Gist instead.

The Gist is both a "how-to" for installing Dockerfile Language Support, as well as an overview of importing/installing packages from TextMate and Sublime Text 3.

TLDR:

You can download the package below (view Gist for instructions) and extract it to your Local #Textastic Directory to install the Language & Syntax Definitions.

Download: dockerfile.tmbundle.zip