Docker 101: A Step-by-Step Guide

Tutorial difficulty level: ✨ made by a blonde, for blondes ✨

Docker in a Nutshell

Docker allows to create, build, and run containerized applications. A container is a standalone package that includes everything an application needs to run, such as code, libraries and dependencies.

To use Docker:

1. Create a DockerFile: this is a text file that contains instructions on how to build a Docker image for an application. The DockerFile specifies the base image to use, the libraries and dependencies to install, and any additional customizations or configurations.

2. Build the Docker image. We can use the docker build command and specify the path to the DockerFile. This will create a Docker image that includes all the ecessary components.
   The basic syntax is:

docker build -t <image-name> <path-to-folder-containing-Dockerfile>

Note. We will build the Docker image starting from a DockerFile which is located in the same directory of the remaining code. However, it is possible to create a Docker image, for example, starting from a Docker image url.
The syntax remains unchanged:

docker build -t <image-name> <url>

A practical example could be:

docker build -t docker_firefox github.com/creack/docker-firefox

3. Run the Docker container. Starting from an image, to create a running Docker container we can use the docker run command, specifying both the name we want to give to the container and the name of the Docker image.
   The basic syntax is:

docker run -d --name=<container-name> <image-name> sleep infinity

4. Execute commands. Now that the container is created, and it is running, we can execute commands in it using the docker exec command..
   The basic syntax is:

docker exec <container-name> <command>

The overall schema is the following:

Docker Tutorial for this Repo

First of all, the DockerFile.

FROM python:3.9
ADD main.py .

ADD requirements.txt /tmp/requirements.txt
RUN pip install -r /tmp/requirements.txt

The FROM line tells us the base image that we want to retrieve to build our new image: in this case it is python 3.9.
The ADD line copies files from the first argument and adds them to the container file system under the second argument path: in this case we copy main.py to . directory. Moreover, we also copy the requirements.txt to a tmp folder. In this way, it will be possible for Docker to access the list of requirements and install them.
The RUN line executes the given command.

Starting from this DockerFile, we build the image with the following syntax:

docker build -t hello_docker .

We can check on Docker Desktop that the image is in fact created.

Now that we have the Docker image, we create a running Docker container with the following syntax:

docker run -d --name=hello_docker_container hello_docker sleep infinity

We can check on Docker Desktop that the container is in fact created.

Now everything is set, and we can finally run what we want inside the Docker container. We run the script main.py with the following syntax:

docker exec hello_docker_container python3 main.py

Docker Extras

Once we are done with everything, we should stop the container from running.
It is possible to do it from Docker Desktop:

Or, we can do it from command line with the following syntax:

docker stop <container-name>

In our specific case, this would be:

docker stop hello_docker_container

---

If we don't need the container or the image, we can delete them both.
Again, this is possible to be done from Docker Desktop:

Or, we can do it from command line with the following syntax:

• To delete the container:

docker rm <container-name>

In our specific case:

docker rm hello_docker_container

• To delete the image:

docker image rm <image-name>

In our specific case:

docker image rm hello_docker

Useful semi-Off-Topic

A good guide to install Docker on Windows.

Moreover, in Docker Desktop settings, make sure to always have this checkbox checked: