Dockerfile: Delete the apt-get lists after installing

This fixes an issue reported by Codacy and Hadolint.

Signed-off-by: Stefan Weil <sw@weilnetz.de>

Dockerfile

@@ -4,7 +4,9 @@ FROM ubuntu
 LABEL maintainer="Ian Blenke <ian@blenke.com>"
 
 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