Docker Run

In the previous sections, we saw how to define the image requirements with the Dockerfile and build it with the build command. This section focuses on running Python inside a container using the docker run command.

Docker run

The docker run or run command enables us to create and run a new container from an image. Typically, the run command is used to launch a dockerized application or server or to execute a code following the below syntax:

docker run [OPTIONS] IMAGE [COMMAND] [ARG...]

For example, we can use the run command with the official Python 3.10 image:

docker run python:3.10 

Surprisingly (or not), nothing happened. To understand that better, we need to go back to the Dockerfile. Generally, images can be used to run: - Server - Application

In both cases, we use the Dockerfile to set and enable launching them during the run time. In the case of a server, we use on the Dockerfile the PORT and CMD commands to set the server’s port on the image and launch the server, respectively. We then use the run command and add the -p (or --publish list) option to map the server’s port with a local port. Similarly, to launch an application, we use the CMD command on the Dockerfile to define the launch command during the run time and use the --interactive and --tty options to launch the container in interactive mode, which enables us to access the application.

Let’s now go back to the python:3.10 image and use the inspect command to check if the CMD command was defined:

> docker inspect python:3.10 | jq '.[] | .Config.Cmd'
[
  "python3"
]

Note: We used the jq library again to parse out from the JSON output the CMD metadata

As you can see, the CMD on the python:3.10 image is set to run the default Python launch command - python3, which launches Python during the run time. Let’s now add the --interactive and --tty options to run the container in an interactive mode:

 docker run --interactive --tty python:3.10 

This launches the default Python version on the image. We can then test it by using the print command to print Hello World!:

Python 3.10.12 (main, Jun 14 2023, 18:40:54) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> print("Hello World!")
Hello World!
>>> 

OK, we have Python running inside a dockerized environment, so why should we not use it? Mainly due to the following reasons: - This is not a development environment, and it is harder (in my mind) to maintain and develop code from the terminal with respect to Python IDEs such as PyCharm or VScode. - By default, the docker run is an ephemeral process, and therefore, your code is lost when you shut down the container.

While there are ways to overcome the above issues, it is still convoluted and not as efficient as using VScode. In the next section, we will see how to set and run Python code with VScode and the Dev Containers extension.