The purpose of these images is to provide the smallest possible images for every major programming language. Read more about it in this Microcontainers blog post.
Use the iron/LANGUAGE:dev images for building/vendoring.
Use the iron/LANGUAGE images for running (much smaller than the dev ones).
Examples for how to use all of these images for each language can be found here: https://github.com/iron-io/dockerworker