Merge pull request #2040 from stweil/docker

Update Dockerfile

Dockerfile

@@ -1,10 +1,12 @@
 # Dockerfile for local Travis build test
 
 FROM ubuntu
-MAINTAINER Ian Blenke <[email protected]>
+LABEL maintainer="Ian Blenke <[email protected]>"
 
 RUN apt-get update
-RUN DEBIAN_FRONTEND=noninteractive apt-get install -y cmake curl git ruby bundler wget unzip
+RUN DEBIAN_FRONTEND=noninteractive apt-get install -y cmake curl git ruby bundler wget unzip \
+  && apt-get clean \
+  && rm -rf /var/lib/apt/lists/*
 RUN gem install bundler travis --no-ri --no-rdoc
 RUN git clone --depth 1 https://github.com/travis-ci/travis-build ~/.travis/travis-build
 RUN bundle install --gemfile ~/.travis/travis-build/Gemfile