install ruby via rvn

Dockerfile

-FROM centos/ruby-26-centos7
-
-RUN scl enable rh-ruby26 "gem install -N fpm-cookery"
-
-USER root
+FROM centos:7
 RUN yum -y install epel-release
 RUN curl -sL https://rpm.nodesource.com/setup_12.x | bash -
 RUN yum -y update
 RUN yum -y install \
     ansible \
+    autoconf \
+    automake \
+    bison \
     gcc \
     gcc-c++ \
     git \
+    libffi-devel \
+    libtool \
     make \
     nodejs \
     openssh-clients \
     openssl \
+    openssl-devel \
+    readline-devel \
     rpm-build \
     rsync \
+    ruby \
+    sqlite-devel \
     wget \
     which \
+    zlib-devel \
  && yum clean all
 
+# Install RVM
+RUN gpg --keyserver hkp://keys.gnupg.net --recv-keys \
+      409B6B1796C275462A1703113804BB82D39DC0E3 \
+      7D2BAF1CF37B13E2069D6956105BD0E739499BDB \
+ && curl -sSL https://get.rvm.io | bash -s -- --autolibs=read-fail stable \
+ && echo 'rvm_silence_path_mismatch_check_flag=1' >> ~/.rvmrc
+
+SHELL ["/bin/bash", "-lc"]
+CMD ["/bin/bash", "-l"]
+
+# Install Ruby
+RUN rvm install 2.7.0 \
+ && rvm alias create 2.7 ruby-2.7.0 \
+ && rvm use --default 2.7.0
+