Make Dockerfile a programming language

[lildude]

Description

Dockerfiles have grown hugely in popularity since first added to Linguist and are essentially scripts. To quote https://docs.docker.com/engine/reference/builder/:

A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image.

... which is essentially what a shell script it. This PR flips the categorisation from data to programming so these files are now included in the language stats.

Checklist:

• I am adding new or changing current functionality
  • I have added or updated the tests for the new or changed functionality.

No need for new or different tests as we're just changing how a language is classified.

TODO:

• Find a colour that is representative, but doesn't conflict - gonna be tough as Docker ❤️ blue, like everyone else.

Fixes #3628

[lildude]

Wow! First try at picking the colour and it's a 🎯, courtesy of https://brandcolors.net/b/docker 🎉

[pchaigno]

LGTM!