[Security] Update Dockerfile for Ruby 2.6.6 (#13393)

Dockerfile

@@ -38,8 +38,8 @@ RUN apt update && \
 	make -j$(nproc) > /dev/null && \
 	make install_bin install_include install_lib
 
-# Install ruby
-ENV RUBY_VER="2.6.5"
+# Install Ruby
+ENV RUBY_VER="2.6.6"
 ENV CPPFLAGS="-I/opt/jemalloc/include"
 ENV LDFLAGS="-L/opt/jemalloc/lib/"
 RUN apt update && \