Docs container runs and serves content successfully

[diverdane]

Summary

Currently, if the secretless-broker docs container is failing with a version mismatch between Ruby and jekyll-sass-converter.
Reference: https://github.com/cyberark/secretless-broker/blob/master/CONTRIBUTING.md#run-in-docker

Steps to Reproduce

To reproduce the failure, run:

cd docs
docker-compose up -d

When the container is built, you will see the following error:

ERROR:  Error installing jekyll:
	There are no versions of jekyll-sass-converter (~> 2.0) compatible with your Ruby & RubyGems. Maybe try installing an older version of the gem you're looking for?
	jekyll-sass-converter requires Ruby version >= 2.4.0. The current ruby version is 2.3.7.456.

Expected Results

Container should start up, and documentation should be served on localhost:4000.

Actual Results (including error logs, if applicable)

See error message above.

Additional Information

Problem can be fixed with the following change:

diff --git a/docs/Dockerfile b/docs/Dockerfile
index b6817fd..976d7a2 100644
--- a/docs/Dockerfile
+++ b/docs/Dockerfile
@@ -1,4 +1,5 @@
-FROM ruby:2.3.7
+FROM ruby:2.4.0
+RUN gem update --system
 RUN gem install bundler jekyll
 RUN mkdir /src
 COPY ./Gemfile** /src/