Use alpine-ruby-builder as base for build stage
Use alpine-ruby-builder as base for build stage

Required for https://github.com/FundingCircle/docker-images/pull/264.

Will reduce the image size by ~150MB.

FundingCircle/shipment_tracker,FundingCircle/shipment_tracker,FundingCircle/shipment_tracker,FundingCircle/shipment_tracker