Build your Python image
Estimated reading time: 11 minutes
- Build images
- Run your image as a container
- Use containers for development
- Configure CI/CD
- Deploy your app
Work through the orientation and setup in Get started Part 1 to understand Docker concepts.
Now that we have a good overview of containers and the Docker platform, let’s take a look at building our first image. An image includes everything needed to run an application - the code or binary, runtime, dependencies, and any other file system objects required.
To complete this tutorial, you need the following:
- Python version 3.8 or later. Download Python
- Docker running locally. Follow the instructions to download and install Docker
- An IDE or a text editor to edit files. We recommend using Visual Studio Code.
Let’s create a simple Python application using the Flask framework that we’ll use as our example. Create a directory in your local machine named
python-docker and follow the steps below to create a simple web server.
$ cd /path/to/python-docker $ pip3 install Flask $ pip3 freeze > requirements.txt $ touch app.py
Now, let’s add some code to handle simple web requests. Open this working directory in your favorite IDE and enter the following code into the
from flask import Flask app = Flask(__name__) @app.route('/') def hello_world(): return 'Hello, Docker!'
Test the application
Let’s start our application and make sure it’s running properly. Open your terminal and navigate to the working directory you created.
$ python3 -m flask run
To test that the application is working properly, open a new browser and navigate to
Switch back to the terminal where our server is running and you should see the following requests in the server logs. The data and timestamp will be different on your machine.
127.0.0.1 - - [22/Sep/2020 11:07:41] "GET / HTTP/1.1" 200 -
Create a Dockerfile for Python
Now that our application is running properly, let’s take a look at creating a Dockerfile.
A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image. When we tell Docker to build our image by executing the
docker build command, Docker reads these instructions and execute them consecutively and create a Docker image as a result.
Let’s walk through creating a Dockerfile for our application. In the root of your working directory, create a file named
Dockerfile and open this file in your text editor.
The name of the Dockerfile is not important but the default filename for many commands is simply
Dockerfile. Therefore, we’ll use that as our filename throughout this series.
The first line to add to the Dockerfile is a
# syntax parser directive.
While optional, this directive instructs the Docker builder what syntax to use
when parsing the Dockerfile, and allows older Docker versions with BuildKit enabled
to upgrade the parser before starting the build. Parser directives
must appear before any other comment, whitespace, or Dockerfile instruction in
your Dockerfile, should be the first line in Dockerfiles.
We recommend using
docker/dockerfile:1, which always points to the latest release
of the version 1 syntax. BuildKit automatically checks for updates of the syntax
before building, making sure you are using the most current version.
Next, we need to add a line in our Dockerfile that tells Docker what base image we would like to use for our application.
# syntax=docker/dockerfile:1 FROM python:3.8-slim-buster
Docker images can be inherited from other images. Therefore, instead of creating our own base image, we’ll use the official Python image that already has all the tools and packages that we need to run a Python application.
To learn more about creating your own base images, see Creating base images.
To make things easier when running the rest of our commands, let’s create a working directory. This instructs Docker to use this path as the default location for all subsequent commands. By doing this, we do not have to type out full file paths but can use relative paths based on the working directory.
Usually, the very first thing you do once you’ve downloaded a project written in Python is to install
pip packages. This ensures that your application has all its dependencies installed.
Before we can run
pip3 install, we need to get our
requirements.txt file into our image. We’ll use the
COPY command to do this. The
COPY command takes two parameters. The first parameter tells Docker what file(s) you would like to copy into the image. The second parameter tells Docker where you want that file(s) to be copied to. We’ll copy the
requirements.txt file into our working directory
COPY requirements.txt requirements.txt
Once we have our
requirements.txt file inside the image, we can use the
RUN command to execute the command
pip3 install. This works exactly the same as if we were running
pip3 install locally on our machine, but this time the modules are installed into the image.
RUN pip3 install -r requirements.txt
At this point, we have an image that is based on Python version 3.8 and we have installed our dependencies. The next step is to add our source code into the image. We’ll use the
COPY command just like we did with our
requirements.txt file above.
COPY . .
COPY command takes all the files located in the current directory and copies them into the image. Now, all we have to do is to tell Docker what command we want to run when our image is executed inside a container. We do this using the
CMD command. Note that we need to make the application externally visible (i.e. from outside the container) by specifying
CMD [ "python3", "-m" , "flask", "run", "--host=0.0.0.0"]
Here’s the complete Dockerfile.
# syntax=docker/dockerfile:1 FROM python:3.8-slim-buster WORKDIR /app COPY requirements.txt requirements.txt RUN pip3 install -r requirements.txt COPY . . CMD [ "python3", "-m" , "flask", "run", "--host=0.0.0.0"]
Just to recap, we created a directory in our local machine called
python-docker and created a simple Python application using the Flask framework. We also used the
requirements.txt file to gather our requirements, and created a Dockerfile containing the commands to build an image. The Python application directory structure would now look like:
python-docker |____ app.py |____ requirements.txt |____ Dockerfile
Build an image
Now that we’ve created our Dockerfile, let’s build our image. To do this, we use the
docker build command. The
docker build command builds Docker images from a Dockerfile and a “context”. A build’s context is the set of files located in the specified PATH or URL. The Docker build process can access any of the files located in this context.
The build command optionally takes a
--tag flag. The tag is used to set the name of the image and an optional tag in the format
name:tag. We’ll leave off the optional
tag for now to help simplify things. If you do not pass a tag, Docker uses “latest” as its default tag. You can see this in the last line of the build output.
Let’s build our first Docker image.
$ docker build --tag python-docker . [+] Building 2.7s (10/10) FINISHED => [internal] load build definition from Dockerfile => => transferring dockerfile: 203B => [internal] load .dockerignore => => transferring context: 2B => [internal] load metadata for docker.io/library/python:3.8-slim-buster => [1/6] FROM docker.io/library/python:3.8-slim-buster => [internal] load build context => => transferring context: 953B => CACHED [2/6] WORKDIR /app => [3/6] COPY requirements.txt requirements.txt => [4/6] RUN pip3 install -r requirements.txt => [5/6] COPY . . => [6/6] CMD [ "python3", "-m", "flask", "run", "--host=0.0.0.0"] => exporting to image => => exporting layers => => writing image sha256:8cae92a8fbd6d091ce687b71b31252056944b09760438905b726625831564c4c => => naming to docker.io/library/python-docker
View local images
To see a list of images we have on our local machine, we have two options. One is to use the CLI and the other is to use Docker Desktop. As we are currently working in the terminal let’s take a look at listing images using the CLI.
To list images, simply run the
docker images command.
$ docker images REPOSITORY TAG IMAGE ID CREATED SIZE python-docker latest 8cae92a8fbd6 3 minutes ago 123MB python 3.8-slim-buster be5d294735c6 9 days ago 113MB
You should see at least two images listed. One for the base image
3.8-slim-buster and the other for the image we just built
As mentioned earlier, an image name is made up of slash-separated name components. Name components may contain lowercase letters, digits and separators. A separator is defined as a period, one or two underscores, or one or more dashes. A name component may not start or end with a separator.
An image is made up of a manifest and a list of layers. Do not worry too much about manifests and layers at this point other than a “tag” points to a combination of these artifacts. You can have multiple tags for an image. Let’s create a second tag for the image we built and take a look at its layers.
To create a new tag for the image we’ve built above, run the following command.
$ docker tag python-docker:latest python-docker:v1.0.0
docker tag command creates a new tag for an image. It does not create a new image. The tag points to the same image and is just another way to reference the image.
Now, run the
docker images command to see a list of our local images.
$ docker images REPOSITORY TAG IMAGE ID CREATED SIZE python-docker latest 8cae92a8fbd6 4 minutes ago 123MB python-docker v1.0.0 8cae92a8fbd6 4 minutes ago 123MB python 3.8-slim-buster be5d294735c6 9 days ago 113MB
You can see that we have two images that start with
python-docker. We know they are the same image because if you take a look at the
IMAGE ID column, you can see that the values are the same for the two images.
Let’s remove the tag that we just created. To do this, we’ll use the
rmi command. The
rmi command stands for remove image.
$ docker rmi python-docker:v1.0.0 Untagged: python-docker:v1.0.0
Note that the response from Docker tells us that the image has not been removed but only “untagged”. You can check this by running the
docker images command.
$ docker images REPOSITORY TAG IMAGE ID CREATED SIZE python-docker latest 8cae92a8fbd6 6 minutes ago 123MB python 3.8-slim-buster be5d294735c6 9 days ago 113MB
Our image that was tagged with
:v1.0.0 has been removed, but we still have the
python-docker:latest tag available on our machine.
In this module, we took a look at setting up our example Python application that we will use for the rest of the tutorial. We also created a Dockerfile that we used to build our Docker image. Then, we took a look at tagging our images and removing images. In the next module we’ll take a look at how to:
python, build, images, dockerfile