Fix cache key used to use base image os

Signed-off-by: Balasankar "Balu" C <balasankar@gitlab.com>

.gitlab-ci.yml

@@ -56,7 +56,7 @@ include:
       files:
         - .gitlab-ci.yml
         - ruby/Gemfile.lock
-      prefix: debian-${DEBIAN_VERSION}-ruby-${RUBY_VERSION}
+      prefix: debian-${BASE_IMAGE_OS}-ruby-${RUBY_VERSION}
     paths:
       - .ruby
     policy: pull