split rubies out into separate image layers

because running in batches is failing too often due to
https://bugs.ruby-lang.org/issues/18506. let's opt for simple and
debuggable.

Dockerfile.mri.erb

@@ -132,32 +132,43 @@ RUN bash -c " \
     "
 
 <%
+# Build xruby versions prior ruby2_keywords using ruby-2.5
+# Build xruby versions with ruby2_keywords using ruby-3.x
+# Note that parallel builds of ruby are often flaky, see https://bugs.ruby-lang.org/issues/18506
 axrubies = if platform =~ /x64-mingw-ucrt/
   [
     # Rubyinstaller-3.1.0+ is platform x64-mingw-ucrt
-    ["3.3.0-preview3:3.2.0:3.1.0", "3.1.3", true],
+    ["3.1.0", "3.1.3", true],
+    ["3.2.0", "3.1.3", true],
+    ["3.3.0-preview3", "3.1.3", true],
   ]
 elsif platform =~ /x64-mingw32/
   [
     # Rubyinstaller prior to 3.1.0 is platform x64-mingw32
-    ["2.6.0:2.5.0:2.4.0", "2.5.9", false],
-    ["3.0.0:2.7.0", "3.1.3", true],
+    ["2.4.0", "2.5.9", true],
+    ["2.5.0", "2.5.9", true],
+    ["2.6.0", "2.5.9", true],
+    ["2.7.0", "3.1.3", true],
+    ["3.0.0", "3.1.3", true],
   ]
 else
   [
-    # Build xruby versions prior ruby2_keywords in parallel using ruby-2.5
-    ["2.6.0:2.5.0:2.4.0", "2.5.9", false],
-    # Build xruby versions with ruby2_keywords in parallel using ruby-3.x
-    ["3.3.0-preview3:3.2.0:3.1.0:3.0.0:2.7.0", "3.1.3", true],
+    ["2.4.0", "2.5.9", true],
+    ["2.5.0", "2.5.9", true],
+    ["2.6.0", "2.5.9", true],
+    ["2.7.0", "3.1.3", true],
+    ["3.0.0", "3.1.3", true],
+    ["3.1.0", "3.1.3", true],
+    ["3.2.0", "3.1.3", true],
+    ["3.3.0-preview3", "3.1.3", true],
   ]
 end
 
 axrubies.each do |xrubies, rvm, parallel| %>
 ENV XRUBIES <%= xrubies %>
 
 <% strip = '-s' if platform !~ /darwin/ %>
-# Build xruby versions in parallel
-# Then cleanup all build artifacts
+# Build xruby versions, then cleanup all build artifacts
 RUN bash -c " \
       rvm use <%= rvm %> && \
       export CPPFLAGS='<%= "-D__USE_MINGW_ANSI_STDIO=1" if platform=~/x64-mingw-ucrt/ %>' && \