Allow specifying of files to check

[ChrisChinchilla]

Description

A start at fixing issue https://github.com/testthedocs/redactor/issues/21 read comments there.

Checklist:

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings

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This change is 

[svx]

@ChrisChinchilla would you mind to update the docs, too ?

ALso I will cleanup the Dockerfile and entrypoint script later and remove old stuff and add better comments, is that OK with you ?.

Other quick question, did you test building and using the new container locally ? :)

[ChrisChinchilla]

@svx Sure, but this is still in progress, so won't update docs yet, see discussion on https://github.com/testthedocs/redactor/issues/21

And it did build locally yes.