[ruby/rack] Use 5 threads for Itsi (#10436)

Using 1 thread seems to make things slower. Try 5 threads similar to
Puma.

Author: p8

frameworks/Ruby/rack/rack-itsi.dockerfile

@@ -19,7 +19,7 @@ RUN bundle install --jobs=8
 
 COPY . .
 
-ENV MAX_THREADS=1
+ENV MAX_THREADS=5
 
 EXPOSE 8080