Mystery of changes not reflected on Docker container

I was recently working on a project where I needed to adjust an existing Dockerfile configuration to add a new database schema.

For a (then) mysterious reason, I couldn’t see the new database schema created in the database container as I had expected. It took me a while to figure this out. I am sharing this hoping this will save time for other developers facing a similar situation.

So what was happening? Why were the Dockerfile changes not reflected on the database container?

To explain this and see it in action, let’s use a small example: https://github.com/taomoh/docker-gotcha1

In this example, we are using a Dockerfile to “dockerise” a basic “Hello World” Python application, exposed on port 8000.

The Docker image built from this Dockerfile will contain all the dependencies the application requires.

1. Clone the github project docker-gotcha1: https://github.com/taomoh/docker-gotcha1

2. Start up the application by running docker compose command from the root folder of the project:

$ docker-compose up -d web

Check the container started as expected:

$ docker ps
CONTAINER ID IMAGE     COMMAND     CREATED       STATUS       PORTS                                     NAMES
97f84f04df98 tip-1_web "flask run" 6 seconds ago Up 3 seconds 0.0.0.0:8000->5000/tcp, :::8000->5000/tcp tip-1_web_1

The first time you run this command, it will build the application Docker image, based on the instructions provided in Dockerfile:

$ docker image ls
REPOSITORY TAG        IMAGE ID     CREATED        SIZE
tip-1_web  latest     268f207b4ba4 19 seconds ago 179MB
python     3.7-alpine caace8ae014d 11 days ago    45.5MB

3. Hit http://localhost:8000/ on the browser: you’ll see the message “Hello World!” as expected.

4. Now let’s change something in the application code:
For example, in app.py, replace: return `Hello World` with return `Hola Mundo!` and restart the Docker container:

$ docker-compose stop
$ docker-compose up -d web

6. Hit http://localhost:8000/ on the browser: you still see the message “Hello World!” and not “Hola Mundo!” as expected. What’s going on?

Well, the docker compose command have re-used the image created from the first run, which still has the old version of the code:

$ docker image ls
REPOSITORY TAG        IMAGE ID     CREATED        SIZE
tip-1_web  latest     268f207b4ba4 17 minutes ago 179MB
python     3.7-alpine caace8ae014d 11 days ago    45.5M

You need to rebuild a fresh Docker image to reflect the code change. To do this you need to pass the --build switch:

$ docker-compose stop
$ docker-compose up -d --build web

List Docker images and you will see that a fresh Docker image of the web application has just been created:

$ docker image ls
REPOSITORY TAG    IMAGE ID     CREATED            SIZE
tip-1_web  latest 5e6982bdd0f1 About a minute ago 179MB
<none>     <none> 268f207b4ba4 23 minutes ago     179M

This time, if you hit http://localhost:8000/ on the browser, you will see the new message “Hola Mundo!” as expected.

P.S. The same principle applies if you had to change Dockerfile: you’ll have to rebuild your application image, by providing the --build switch to the docker compose up command.

About the author:
Taoufik Mohdit is a Senior Java Developer here at Version 1.