Extending Openshift’s source to image

Openshift 3 introduces a new concept “source to image” or s2i. It’s a way to create a Docker image out of some source code and a Docker image – for example a python s2i image. It makes Openshift 3’s docker based workflow feel more like Openshift 2 or Heroku.

Image from http://blog.octo.com/en/openshift-3-private-paas-with-docker/

One problem I ran into using s2i-python was a lack of binary packages installed that are needed to build things like Pillow. For this we need to extend the base image to add what we want. You can see the end result on github.

Let’s review the Dockerfile I build. We start by extending centos’s s2i image. Note I’m using Python 3.4. You can review the upstream project here to see what I’m extending.

FROM centos/python-34-centos7

Next I’m mimicking how centos installs packages from the upstream file. Upstream and my version

You can see I installed postgresql client tools and node too which I need. I also install a couple pip packages I know I’ll need on all my python projects – this is done just to speed up the build time. Do as much or as little as you want.

Next I’ll post it on Docker hub with automated builds. See here. I added centos/python-34-centos7 as a linked repository. This way my docker image builds any time the upstream image builds too – ensuring I get security updates.

Screenshot from 2016-03-02 15-40-55

Finally I just use the image as I would the original s2i python in openshift. I can add it as I would any image with oc import-image s2i-python --from="thelab/s2i-python:latest" --confirm

Next post I’ll describe how to reuse a customized s2i python image on local development with Docker compose.

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