Merge branch 'update-docker-in-ruby-docker' into 'master'

Use official docker package in ruby_docker image

See merge request !51

docker/Dockerfile_ruby_docker

@@ -18,7 +18,9 @@ RUN echo "deb http://ftp.debian.org/debian jessie-backports main" >> /etc/apt/so
         gettext \
         libssl-dev \
         libc6-dev \
-        docker.io \
+        apt-transport-https \
+        gnupg2 \
+        software-properties-common \
         && apt-get clean \
         && rm -rf /var/lib/apt/lists/*
 
@@ -30,4 +32,9 @@ RUN curl -fsSL "https://cache.ruby-lang.org/pub/ruby/2.3/ruby-${RUBY_VERSION}.ta
   && make \
   && make install
 
+RUN curl -fsSL https://download.docker.com/linux/debian/gpg | apt-key add - \
+&& add-apt-repository  "deb [arch=amd64] https://download.docker.com/linux/debian $(lsb_release -cs) stable" \
+&& apt-get update -q \
+&& apt-get install -yq docker-ce
+
 RUN gem install bundler