Add Dockerfile for ROR generator

[wing328]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

• add Dockerfile
• minor fix to mustache templates

Credits go to @ledsun who provides the Dockerfile and docker-entrypoint.sh

To close #713

[wing328]

Test result looks good locally:

=> Booting Puma
=> Rails 5.0.7 application starting in development on http://0.0.0.0:3000
=> Run `rails server -h` for more startup options
Puma starting in single mode...
* Version 3.12.0 (ruby 2.5.1-p57), codename: Llamas in Pajamas
* Min threads: 5, max threads: 5
* Environment: development
* Listening on tcp://0.0.0.0:3000
Use Ctrl-C to stop
^C- Gracefully stopping, waiting for requests to finish
=== puma shutdown: 2018-08-02 11:18:43 +0000 ===
- Goodbye!
Exiting

[wing328]

cc @cliffano @zlx @ledsun