Add Dockerfile to .editorconfig and do a bit of housekeeping on the file (#1049)

This PR makes some very minor updates to `.editorconfig`:

- defines settings for Dockerfile
- remove pattern for `*.BUILD` because this repo has no such files
- streamline the comments some more

Author: mhucka

.editorconfig

@@ -15,11 +15,10 @@
 
 root = true
 
-# We follow the TensorFlow coding style
-# https://www.tensorflow.org/community/contribute/code_style
+# We follow TF's style: https://tensorflow.org/community/contribute/code_style
 
-# IMPORTANT: some of the other config files (.clang-format, etc.) also have
-# the same settings and need to be updated if changes are made to this file.
+# IMPORTANT: some of the other config files (e.g., .pylintrc) also have settings
+# for the same properties & need to be updated if changes are made to this file.
 [*]
 charset = utf-8
 indent_style = space
@@ -28,7 +27,10 @@ spelling_language = en-US
 trim_trailing_whitespace = true
 max_line_length = 80
 
-[{BUILD,*.BUILD,*.bzl,*.bazel,.bazelrc,WORKSPACE}]
+[{BUILD,*.bzl,*.bazel,.bazelrc,WORKSPACE}]
+indent_size = 4
+
+[Dockerfile]
 indent_size = 4
 
 [{*.cc,*.h}]