Docker by Example: A simple container

In the docker-by-example files (see note above), navigate in your terminal into the 1-python-basic folder.

This one's not super useful, but if you have a Python program you need to run, but can't install Python (but you have Docker?), you can can run Python in Docker.

First you have to build the image. This step will take a while the first time, as you have to download some base images from the Docker Hub, then build them all locally. (Note the . at the end of the command. That's important. It tells the Docker command to look for a Dockerfile in the current directory)

docker build -t python-basic:v1 .

Then you can tell Docker to create a container from the image and run that container.

docker run python-basic:v1

That's it. You've run your first container. It's not super fancy, but it runs.

Immutability

One of the things to remember about Docker (and containers) is that the containers and images are immutable. That means that once created, they don't change. You can overwrite an image with a new image, but otherwise the images you create don't change. Let's see that in action.

Open app.py and change "Hello world" to something else like "Hello there". Now that we've changed app.py, run the container again:

docker run python-basic:v1

That should do the same thing it did last time. We changed the code, but we didn't change the image, even though the code is run by the image. If you look at the Dockerfile, you'll see that copying app.py into the container image is one of the steps. That copying happens when you build the image, not when you run it.

Let's create a new Docker image using the new app.py. Notice that I use the tag v2 instead of v1 this time. This create a new image, and lets us keep the old one around for later use.

docker build -t python-basic:v2 .

Now you can run the new container with

docker run python-basic:v2

AND, you can still run a container from the old image with

docker run python-basic:v1

This immutability can be very useful, especially in a situation where you want to roll back to a previous working version of your code.

Next up

Docker by Example: Web app containers

💡This tutorial, and the others in this series, use files that you can access from https://github.com/rschuetzler/docker-by-example/. Click the green Code button, then download and unzip the file on your computer to get started. Open up the 2-node-app folder (in your text editor and in your terminal)

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