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After building the image:

docker build -t hoppus/counter .

I attempt to run the image with:

docker run -p 4567:4567 --link redis:redis hoppus/counter

I receive the following error:

/usr/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- json (LoadError)
    from /usr/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /usr/lib/ruby/gems/2.4.0/gems/rack-2.0.4/lib/rack/session/cookie.rb:6:in `<top (required)>'
    from /usr/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /usr/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /usr/lib/ruby/gems/2.4.0/gems/sinatra-2.0.1/lib/sinatra/base.rb:1775:in `<class:Base>'
    from /usr/lib/ruby/gems/2.4.0/gems/sinatra-2.0.1/lib/sinatra/base.rb:894:in `<module:Sinatra>'
    from /usr/lib/ruby/gems/2.4.0/gems/sinatra-2.0.1/lib/sinatra/base.rb:22:in `<top (required)>'
    from /usr/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /usr/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /usr/lib/ruby/gems/2.4.0/gems/sinatra-2.0.1/lib/sinatra/main.rb:1:in `<top (required)>'
    from /usr/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /usr/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /usr/lib/ruby/gems/2.4.0/gems/sinatra-2.0.1/lib/sinatra.rb:1:in `<top (required)>'
    from /usr/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:133:in `require'
    from /usr/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:133:in `rescue in require'
    from /usr/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:39:in `require'
    from counter.rb:2:in `<main>'

The fix seems to be to adjust line 3 of the Dockerfile to read:

RUN apk update && apk add ruby ruby-json