Use Dockerfile lexer in code examples (triple backtick label) #5585
Description
Jekyll has finally adopted a version of Rouge greater than 1.7, and this support has cascaded up to the GitHub Pages gem, now pegged at 2.2.1:
https://pages.github.com/versions/
Recently, we revved the Gemfile in production to GHPages 172, enabling this support
https://github.com/docker/docker.github.io/blob/master/Gemfile
Which is all a long way of saying that dockerfile (or the docker alias) should be the label we use in code fences, where conf is what we use currently.
```dockerfile`
Stuff
...
(Except without the extra ` after "Dockerfile")
conf has worked pretty well -- the color coding on Dockerfiles that use conf highlighting generally looks okay: https://docs.docker.com/get-started/part2/#dockerfile
However, there are very likely cases where it breaks down and using native support is obviously going to be better.