`lang:Dockerfile` not working as expected (does not find `Dockerfile`s)

Created by: NiclasvanEyk

• Sourcegraph version: Sourcegraph.com
• Platform information: MacOS Big Sur 11.4, Firefox 91.0.2 (but I don't think this is platform dependant)

Steps to reproduce:

1. Execute a search using the lang:Dockerfile filter, e.g. for the sourcegraph/sourcegraph repository on Github
2. Use the same search, but exchange the lang:Dockerfile filter for file:Dockerfile, e.g. the same example as in search 1 with exchanged filters

Expected behavior:

Search (1) and (2) should yield similar results, especially for files called Dockerfile.

Actual behavior:

Search (1) does not yield any results, while search (2) correctly finds all Dockerfiles.

It seems like the lang:Dockerfile filter only takes the file extension into account. For example, when I do not limit the search to a particular repository and apply lang:Dockerfile (link), only files like something.Dockerfile, so ones containing the .Dockerfile suffix are returned. This is even more obvious when also applying file:^Dockerfile$ (link), which yields no results.

Additional notes

The syntax highlighting seems to have similar issues, just the other way around: A file called Dockerfile is highlighted correctly, while a file with a .Dockerfile suffix does not get highlighted at all.

Also other languages with similar naming conventions suffer from such issues. A search containing lang:Makefile only yields files with a .mk extension, but most makefiles will be just named Makefile, which are found when using file:^Makefile$, but applying both lang:Makefile and file:^Makefile$ yields no results.

Given that Dockerfile is the convention/default filename used by the docker cli, I think that a search containing lang:Dockerfile should also return those files as a result.