What Tripped Me Up When Building My Own Dockerfile

I got stuck pushing a Docker image to GitHub Container Registry (ghcr.io) and then building and using another Docker image that references it, so I’m making a note.

Setup

Base Docker image

• ghcr.io/username/child:latest

FROM python:3.8-slim

RUN echo "foo"

Docker image that references the base Docker image

• ghcr.io/username/parent:latest

FROM ghcr.io/username/child:latest

# install various things

The problem

The Docker image used in production

FROM ghcr.io/username/parent:latest

# build various things
RUN python -v

In this state, checking the Python version shows 3.8. To update the Python version, I updated the Dockerfile for ghcr.io/username/child:latest.

+ FROM python:3.9-slim

RUN echo "foo"

When I rebuilt the production Docker image, the Python version was still 3.8.

The cause

Even if you set the base image to latest, it does not always pull the newest image. It pulls the latest image as it was at the time that Dockerfile was built. So when you update the base Docker image, you also need to rebuild the Docker image that references the base Docker image. It’s obvious in hindsight, but it was the first time it caught me out.

Summary

The last place you look.