logic explicitly refers to manylinux

instead of the obscure `platform =~ /linux/`

Dockerfile.mri.erb

@@ -1,11 +1,14 @@
-FROM <%=
-  case platform
-    when /x86_64-linux/ then "quay.io/pypa/manylinux2014_x86_64"
-    when /x86-linux/ then "quay.io/pypa/manylinux2014_i686"
-    else "ubuntu:20.04"
-  end %>
-
-<% if platform=~/linux/ %>
+<%
+image = case platform
+  when /x86_64-linux/ then "quay.io/pypa/manylinux2014_x86_64"
+  when /x86-linux/ then "quay.io/pypa/manylinux2014_i686"
+  else "ubuntu:20.04"
+end
+manylinux = !!(image =~ manylinux)
+%>
+FROM <%= image %>
+
+<% if manylinux %>
 # install packages which rvm will require
 RUN yum install -y autoconf gcc-c++ libtool readline-devel sqlite-devel ruby openssl-devel xz cmake sudo less
 
@@ -72,7 +75,7 @@ RUN sudo mkdir -p /usr/local/rake-compiler && \
 # Add cross compilers for Windows and Linux
 USER root
 
-<% if platform !~ /linux/ %>
+<% unless manylinux %>
 RUN apt-get -y update && \
     apt-get install -y <%
 if platform=~/darwin/     %> clang python lzma-dev libxml2-dev libssl-dev libc++-10-dev <% end %><%
@@ -85,7 +88,7 @@ RUN bash -c " \
         rvm alias create default 2.5.8 && \
         rvm use default "
 
-<% if platform=~/linux/   %>
+<% if manylinux %>
 # Create dev tools x86-linux-*
 COPY build/mk_i686.rb /root/
 RUN bash -c " \
@@ -200,7 +203,7 @@ RUN printf "1\n" | update-alternatives --config <%= target %>-gcc && \
     printf "1\n" | update-alternatives --config <%= target %>-g++
 <% end %>
 
-<% if platform=~/linux/ %>
+<% if manylinux %>
 # Enable modern compiler toolset of manylinux image
 RUN echo "export PATH=\$DEVTOOLSET_ROOTPATH/usr/bin:\$PATH" >> /etc/rubybashrc
 

Rakefile

@@ -26,7 +26,7 @@ namespace :build do
       sh "docker build -t #{DOCKERHUB_USER}/rake-compiler-dock-mri-#{platform}:#{RakeCompilerDock::IMAGE_VERSION} -f Dockerfile.mri.#{platform} ."
     end
 
-    df = ERB.new(File.read("Dockerfile.mri.erb")).result(binding)
+    df = ERB.new(File.read("Dockerfile.mri.erb"), trim_mode: ">").result(binding)
     File.write(sdf, df)
     CLEAN.include(sdf)
   end