add Docker image to help test with ngx_mruby

[vessi]

Add Dockerfile to build with ngx_mruby from @matsumoto-r

[matsumotory]

👀

[vessi]

Look like I need to tell something why I'm doing this mongo driver. It's because I noticed that in TechEmpower Framework Benchmark all Ruby frameworks use MySQL as database. And I was impressed with ngx_mruby results so I want to try improve them with adding MongoDB support.

[matsumotory]

Thank you for telling me the reason! I will gladly use mruby-mongo in mod_mruby, ngx_mruby or trusterd.