Remove mention of "apt-get clean" as it's performed automatically

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>

docs/articles/dockerfile_best-practices.md

@@ -217,7 +217,6 @@ recommendations.
         ruby1.9.1 \
         ruby1.9.1-dev \
         s3cmd=1.1.* \
-     && apt-get clean \
      && rm -rf /var/lib/apt/lists/*
 
 The `s3cmd` instructions specifies a version `1.1.0*`. If the image previously
@@ -230,6 +229,9 @@ keep the image size down. Since the `RUN` statement starts with
 `apt-get update`, the package cache will always be refreshed prior to
 `apt-get install`.
 
+> **Note**: The official Debian and Ubuntu images [automatically run `apt-get clean`](https://github.com/docker/docker/blob/03e2923e42446dbb830c654d0eec323a0b4ef02a/contrib/mkimage/debootstrap#L82-L105),
+> so explicit invocation is not required.
+
 ### CMD
 
 [Dockerfile reference for the CMD instruction](../reference/builder.md#cmd)