Remove docker_ file glob from Docker lexer (#1550)

In addition to adding the `*.Dockerfile` file glob, #1059 added a file
glob for `Dockerfile_*` to address #1070. However, as noted by #1539,
this causes issues for files that begin with `dockerfile_` but that are
not Dockerfiles.

Adding the `Dockerfile_*` file glob was an error and this commit
removes it. It is a non-standard name and is not recognised by other
syntax highlighting libraries (e.g. Pygments, Chroma, Linguist). The
correct fix for the issue that precipitated #1070 is to use a
`.gitattributes` file.

lib/rouge/lexers/docker.rb

@@ -8,7 +8,7 @@ class Docker < RegexLexer
       desc "Dockerfile syntax"
       tag 'docker'
       aliases 'dockerfile'
-      filenames 'Dockerfile', '*.Dockerfile', '*.docker', 'Dockerfile_*'
+      filenames 'Dockerfile', '*.Dockerfile', '*.docker'
       mimetypes 'text/x-dockerfile-config'
 
       KEYWORDS = %w(

spec/lexers/docker_spec.rb

@@ -11,7 +11,6 @@
       assert_guess :filename => 'Dockerfile'
       assert_guess :filename => 'docker.docker'
       assert_guess :filename => 'some.Dockerfile'
-      assert_guess :filename => 'Dockerfile_some_extension'
     end
 
     it 'guesses by mimetype' do