Remove apt lists in our docs Dockerfile for consistency :)

Dockerfile

@@ -2,7 +2,7 @@ FROM tianon/mojo
 
 RUN cpanm Term::ReadKey
 
-RUN apt-get update && apt-get install -y vim
+RUN apt-get update && apt-get install -y vim && rm -rf /var/lib/apt/lists/*
 
 COPY . /usr/src/docker-library-docs
 WORKDIR /usr/src/docker-library-docs