docs: Use github syntax tag for docker

Fortunately, it seems like rouge should support it as an alias.

README.adoc

@@ -53,7 +53,7 @@ NOTE: All of the example outputs above were last generated/updated on May 3rd 20
 
 == Usage
 Stop doing this:
-[source, docker]
+[source, dockerfile]
 ----
 FROM ubuntu-debootstrap:14.04
 RUN apt-get update -q \
@@ -64,7 +64,7 @@ ENTRYPOINT ["mysql"]
 ----
 This took 28 seconds to build and yields a 169 MB image.
 Start doing this:
-[source, docker]
+[source, dockerfile]
 ----
 FROM alpine:3.9
 RUN apk add --no-cache mysql-client

docs/usage.adoc

@@ -10,7 +10,7 @@ The `--no-cache` argument prevents `apk` from caching the package index - someth
 
 === Example
 This is an example of a complete Dockerfile for `nginx`:
-[source, docker]
+[source, dockerfile]
 ----
 FROM alpine
 
@@ -24,7 +24,7 @@ CMD ["nginx", "-g", "daemon off;"]
 Another great `apk` feature is the concept of user-defined virtual packages.
 Packages added under a virtual package name can later be removed as one group.
 An example use-case would be removing all build dependencies at once, as in this example:
-[source, docker]
+[source, dockerfile]
 ----
 FROM alpine