Relay Docker Image tag to DockerFile during build phase

In Agile world, we started seeing many of the programming languages release a newer version of it every other week/month.

You might be surprised to see number of nodejs versions released over past year.
https://nodejs.org/en/download/releases/
never ending

So how do we generate a Dockerfile based on nodejs that supports all of its versions ?

  • Make a separate folder for each nodejs version and create DockerFile inside each of it.

Below was our project with multiple folders

Nodejs Folders

This solution might work but what if you have duplicate code in all these folders and you found a bug in it

In one of our project, We currently have multiple folders with same content except a change in tag name used to get that particular nodejs version. It might get harder to manage/change all these files with more nodejs versions to be supported in future.

I did some research and found that hooks can be utilized to eliminate such scenarios and pass tag of main image into DockerFile during build at Docker Hub.

Here is a snippet of my Dockerfile at root folder. I have intentionally not defined a tag. This would be automatically replaced later.

FROM node:{{tag}}

COPY startup /opt/startup  
COPY hostingstart.html /app/hostingstart.html  
# more content .....

ENTRYPOINT ["/opt/startup/init_container.sh"]
  • create hooks folder in project root
  • create build file inside hooks and add below content in it
#!/usr/bin/env bash

# DOCKER_TAG would be replaced by tag in docker hub
sed -i 's@{{tag}}@'"${DOCKER_TAG}"'@' Dockerfile  
docker build -t ${IMAGE_NAME} .  

Sample Project: https://github.com/prashanthmadi/node

With this approach, now i can define tags at Docker hub and pass that to Dockerfile instead of maintaining multiple folders of same content.

Future Improvements:

With the existing approach, i'm using sed to replace tag as dockerhub uses 15.03 and doesn't support using image_tag in FROM directly.

using image_tag in FROM would be supported in docker >15.05 and we might not need sed operation in future.

Reference:
  • Support for environment variables for building containers #6822