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Pin versions in gem install

Problematic code:

FROM ruby:2
RUN gem install bundler

Correct code:

FROM ruby:2
RUN gem install bundler:1.1

Rationale:

https://docs.docker.com/engine/articles/dockerfile_best-practices/

Version pinning forces the build to retrieve a particular version regardless of what’s in the cache. This technique can also reduce failures due to unanticipated changes in required packages.