Change the RUBY_VERSION environment variable in the Dockerfile to X.X.X
Change the RUBY_BASE_VERSION environment variable in the Dockerfile to X.X
Visit https://www.ruby-lang.org/en/downloads/ to get the sha256 hash for the
version you are trying to build and update the RUBY_SHA256 environment
variable in the Dockerfile to that value
Run docker build -t skio/ruby . and make sure that the image builds
Make sure you're on the master branch and run
`git commit -m "Update to Ruby version X.X.X"