use centos ruby25 image and do some cleanup

Dockerfile

-FROM centos:7
+FROM centos/ruby-25-centos7
+
+RUN scl enable rh-ruby25 "gem install -N fpm-cookery"
+
+USER root
+RUN yum -y install epel-release
+RUN curl -sL https://rpm.nodesource.com/setup_10.x | bash -
+RUN yum -y update
+RUN yum -y install \
+    ansible \
+    gcc \
+    gcc-c++ \
+    git \
+    make \
+    nodejs \
+    openssh-clients \
+    openssl \
+    rpm-build \
+    rsync \
+    wget \
+    which \
+ && yum clean all
 
-RUN yum -y update \
-    && yum -y install ruby \
-    && curl -sL https://rpm.nodesource.com/setup_10.x | bash - \
-    && yum -y install nodejs epel-release openssl rpm-build rsync which git \
-    && yum -y install ruby-devel gcc gcc-c++ make rubygems rake openssh-clients wget \
-    && yum -y install ansible \
-    && gem install -N fpm-cookery \
-    && yum -y update && yum clean all
-    
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