bump docker image to 2.3.1

resolves the issue in which the gemfile that bundler is using is the
user's project instead of this project's. This is due to the workdir
being set during action runs.

action.yml

@@ -23,7 +23,7 @@ inputs:
       origin/master
 runs:
   using: docker
-  image: "docker://docker.io/adwerx/pronto-ruby:a87e9fb"
+  image: "docker://docker.io/adwerx/pronto-ruby:2.3.1"
   args:
     - run
     - -r