docs: update code-hints for compatibility with "rouge"

Unlike GitHub's web-UI, the "rouge" hightlighter used in our
online documentation is case-sensitive. As a result, code-blocks
having the Dockerfile (uppercase) code-hint were not highlighted.

This changes those to use lowercase, which is supported by both.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/build.md

@@ -487,7 +487,7 @@ When building a Dockerfile with multiple build stages, `--target` can be used to
 specify an intermediate build stage by name as a final stage for the resulting
 image. Commands after the target stage will be skipped.
 
-```Dockerfile
+```dockerfile
 FROM debian AS build-env
 ...
 
@@ -555,7 +555,7 @@ desired files to a new scratch stage with [`COPY --from`](../builder.md#copy).
 The example `Dockerfile` below uses a separate stage to collect the
 build-artifacts for exporting:
 
-```Dockerfile
+```dockerfile
 FROM golang AS build-stage
 RUN go get -u github.com/LK4D4/vndr
 
@@ -718,7 +718,7 @@ true
 
 The following is an example of docker build with `--squash` argument
 
-```Dockerfile
+```dockerfile
 FROM busybox
 RUN echo hello > /hello
 RUN echo world >> /hello

docs/reference/commandline/pull.md

@@ -167,7 +167,7 @@ Status: Downloaded newer image for ubuntu@sha256:45b23dee08af5e43a7fea6c4cf9c25c
 
 Digest can also be used in the `FROM` of a Dockerfile, for example:
 
-```Dockerfile
+```dockerfile
 FROM ubuntu@sha256:45b23dee08af5e43a7fea6c4cf9c25ccf269ee113168c19722f87876677c5cb2
 MAINTAINER some maintainer <maintainer@example.com>
 ```