Unable to recognise Dockerfiles with extensions

## Preliminary Steps

Please confirm you have...
- [x] reviewed [How Linguist Works](https://github.com/github/linguist#how-linguist-works),
- [x] reviewed the [Troubleshooting](https://github.com/github/linguist#troubleshooting) docs,
- [x] considered implementing an [override](https://github.com/github/linguist#overrides),
- [x] verified an issue has not already been logged for your issue ([linguist issues](https://github.com/issues?utf8=%E2%9C%93&q=is%3Aissue+repo%3Agithub/linguist)).

## Problem Description
We can name a dockerfile multiple ways. Usual way is to name it as it is either `Dockerfile` or `dockerfile` in this case GitHub recognizes a dockerfile. But it's a usual practice that developers name them with extensions like `Dockerfile.dev` or `Dockerfile.prod` or similar to it. In these case GitHub fails to recognize them as Dockerfiles.

### URL of the affected repository:
All repos using Dockerfiles with extensions.
examples:
https://github.com/kmehant/foundation.mozilla.org/blob/master/dockerfiles/Dockerfile.node
### Last modified on:

### Expected language:

Dockerfile
### Detected language:

nothing