Dockerfile for the Ruby-Sinatra generator

[YasuhiroABE]

I add a Dockerfile to the ruby-sinatra generator, and also update the README.md file.
To prepare these files, I needed to update the Gemfile.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master, 5.1.x, 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[wing328]

@YasuhiroABE thanks for the PR

cc @cliffano (2017/07) @zlx (2017/09) @autopp (2019/02)

[wing328]

CircleCI failure not related to this PR.

Tested locally and the result is good.

[wing328]

Samples updated via 9256108