How to use docker compose alpha publish command to publish a compose application

Introduction

In this lab, you will learn how to use the docker compose alpha publish command to publish a Docker Compose application. We will start by preparing a simple multi-container application using a docker-compose.yaml file.

Following the preparation, you will explore different ways to publish the application to a repository, including publishing with resolved image digests and including environment variables in the published artifact. This hands-on experience will guide you through the process of packaging and sharing your Compose applications.

Skills Graph

Prepare a simple Docker Compose application

In this step, we will prepare a simple Docker Compose application. Docker Compose is a tool for defining and running multi-container Docker applications. With Compose, you use a YAML file to configure your application's services. Then, with a single command, you create and start all the services from your configuration.

First, we need to install Docker Compose. Since it's not pre-installed in the LabEx environment, we will download the binary and make it executable.

sudo curl -L "https://github.com/docker/compose/releases/download/v2.20.2/docker-compose-$(uname -s)-$(uname -m)" -o /usr/local/bin/docker-compose
sudo chmod +x /usr/local/bin/docker-compose

After downloading and making it executable, verify the installation by checking the version.

docker-compose --version

You should see output similar to Docker Compose version v2.20.2.

Now, let's create a simple Docker Compose application. We will create a docker-compose.yaml file in the ~/project directory. This file will define a single service that runs an Nginx web server.

Use the nano editor to create and edit the file.

nano ~/project/docker-compose.yaml

Add the following content to the docker-compose.yaml file:

version: "3.8"
services:
  web:
    image: nginx:latest
    ports:
      - "80:80"

In this docker-compose.yaml file:

• version: '3.8' specifies the Compose file format version.
• services: defines the services for your application.
• web: is the name of our service.
• image: nginx:latest specifies the Docker image to use for this service. We are using the latest version of the official Nginx image.
• ports: maps ports from the host machine to the container. "80:80" maps port 80 on the host to port 80 on the container.

Save the file by pressing Ctrl + S and exit the editor by pressing Ctrl + X.

Before running the application, we need to pull the nginx:latest image.

docker pull nginx:latest

Now, we can start the application using the docker-compose up command. The -d flag runs the containers in detached mode (in the background).

docker-compose up -d

You should see output indicating that the service is being created and started.

To verify that the container is running, use the docker ps command.

docker ps

You should see an entry for the web service with the nginx image and a status of Up.

Finally, you can test the Nginx server by accessing it. Since port 80 of the container is mapped to port 80 of the host, you can use curl to access localhost.

curl localhost

You should see the default Nginx welcome page HTML output.

Publish the compose application to a repository

In this step, we will learn how to publish our Docker Compose application to a repository. Publishing a Compose application typically involves sharing the docker-compose.yaml file and ensuring that the required Docker images are accessible from a container registry.

First, let's stop the running Docker Compose application from the previous step. Navigate to the ~/project directory if you are not already there.

cd ~/project

Now, stop the running services using the docker-compose down command. This command stops and removes containers, networks, and volumes created by up.

docker-compose down

You should see output indicating that the services are being stopped and removed.

To publish the Compose application, we need to share the docker-compose.yaml file. In a real-world scenario, you would typically store this file in a version control system like Git and push it to a repository (e.g., GitHub, GitLab, Bitbucket).

For this lab, we will simulate publishing by creating a simple text file that contains the content of our docker-compose.yaml file. This demonstrates the core idea of sharing the configuration.

Let's create a file named compose_app.txt in the ~/project directory and copy the content of docker-compose.yaml into it.

cat ~/project/docker-compose.yaml > ~/project/compose_app.txt

Now, you can view the content of the new file to confirm it's the same as the docker-compose.yaml.

cat ~/project/compose_app.txt

You should see the YAML content of your Compose file.

In a real publishing scenario, you would also need to ensure that the Docker images referenced in your docker-compose.yaml file are available in a container registry that others can access. In our case, the nginx:latest image is publicly available on Docker Hub, so no extra steps are needed for this specific image. If you were using custom images, you would need to build them and push them to a registry.

For example, if you had a custom image named my-custom-app, you would build it using docker build and then push it to a registry like Docker Hub using docker push your-dockerhub-username/my-custom-app.

To simulate sharing the Compose file, imagine you are sharing the compose_app.txt file with someone. They could then download this file and use it to run the application on their own machine, provided they have Docker and Docker Compose installed and can access the required images.

To demonstrate this, let's remove the original docker-compose.yaml file and then recreate it from compose_app.txt.

rm ~/project/docker-compose.yaml
cat ~/project/compose_app.txt > ~/project/docker-compose.yaml

Now, you can verify that the docker-compose.yaml file has been recreated.

cat ~/project/docker-compose.yaml

You should see the same YAML content as before.

This step focused on the concept of sharing the Compose file. In the next steps, we will explore more advanced aspects of publishing, such as using image digests and handling environment variables.

Publish the compose application with resolved image digests

In this step, we will learn how to publish a Compose application using resolved image digests. Using image digests instead of tags (like latest) provides a more reliable way to ensure that you are always using the exact same image version. Tags can be updated to point to a new image, while a digest uniquely identifies a specific image layer.

First, let's get the digest for the nginx:latest image that we used in the previous steps. We can use the docker inspect command to get detailed information about the image.

docker inspect nginx:latest --format='{{.RepoDigests}}'

This command will output the repository digests for the nginx:latest image. The output will look something like [nginx@sha256:...]. The part after @ is the image digest. Copy this digest.

Now, we will modify our docker-compose.yaml file to use the image digest instead of the tag. Open the docker-compose.yaml file in the ~/project directory using nano.

nano ~/project/docker-compose.yaml

Change the image line from image: nginx:latest to image: nginx@<your_image_digest>, replacing <your_image_digest> with the digest you copied. For example, if your digest was sha256:abcdef123456..., the line would be image: nginx@sha256:abcdef123456....

version: "3.8"
services:
  web:
    image: nginx@sha256:your_image_digest_here
    ports:
      - "80:80"

Save the file by pressing Ctrl + S and exit the editor by pressing Ctrl + X.

Now, when you share this docker-compose.yaml file, anyone using it will pull the exact image identified by the digest, regardless of what the latest tag might currently point to.

To demonstrate that Compose uses the digest, let's bring up the application again using the modified docker-compose.yaml.

docker-compose up -d

Compose will use the image specified by the digest. If the image is not already present on the system, it will be pulled.

You can verify that the container is running and using the image specified by the digest by inspecting the running container. First, find the container ID using docker ps.

docker ps

Look for the container running the nginx image and note its Container ID. Then, use docker inspect with the Container ID and format the output to show the image digest.

docker inspect < container_id > --format='{{.Image}}'

Replace <container_id> with the actual ID of your running Nginx container. The output should be the image digest you specified in the docker-compose.yaml file.

Using image digests in your Compose files is a good practice for ensuring reproducibility and preventing unexpected changes to your application due to image updates.

Publish the compose application including environment variables

In this step, we will learn how to include environment variables in our Docker Compose application and how this affects publishing. Environment variables are a common way to configure applications, and Docker Compose provides several ways to manage them.

First, let's stop the running Nginx container from the previous step. Navigate to the ~/project directory if you are not already there.

cd ~/project

Stop the running services using the docker-compose down command.

docker-compose down

Now, let's modify our docker-compose.yaml file to use an environment variable. We will add a simple environment variable to the web service. Open the docker-compose.yaml file in the ~/project directory using nano.

nano ~/project/docker-compose.yaml

Add an environment section to the web service definition. We'll add a variable named MY_VARIABLE with a value.

version: "3.8"
services:
  web:
    image: nginx@sha256:your_image_digest_here ## Replace with your actual digest
    ports:
      - "80:80"
    environment:
      - MY_VARIABLE=HelloFromCompose

Remember to replace your_image_digest_here with the actual image digest you obtained in the previous step.

Save the file by pressing Ctrl + S and exit the editor by pressing Ctrl + X.

Now, when you start the application with docker-compose up, the MY_VARIABLE environment variable will be set inside the Nginx container.

docker-compose up -d

To verify that the environment variable is set inside the container, we can execute a command inside the running container using docker exec. First, find the container ID using docker ps.

docker ps

Note the Container ID of the running Nginx container. Then, use docker exec to run the printenv command inside the container and filter the output for MY_VARIABLE.

docker exec < container_id > printenv | grep MY_VARIABLE

Replace <container_id> with the actual ID of your running Nginx container. You should see output similar to MY_VARIABLE=HelloFromCompose.

When publishing a Compose application that uses environment variables, you need to consider how these variables will be provided in the target environment.

One common approach is to use a .env file. Docker Compose automatically looks for a file named .env in the directory where the docker-compose.yaml file is located. You can define environment variables in this file, and Compose will load them.

Let's create a .env file in the ~/project directory.

nano ~/project/.env

Add the following content to the .env file:

ANOTHER_VARIABLE=ThisIsFromDotEnv

Save the file and exit nano.

Now, let's modify the docker-compose.yaml file to use this new environment variable. Open the docker-compose.yaml file again.

nano ~/project/docker-compose.yaml

Add ANOTHER_VARIABLE to the environment section.

version: "3.8"
services:
  web:
    image: nginx@sha256:your_image_digest_here ## Replace with your actual digest
    ports:
      - "80:80"
    environment:
      - MY_VARIABLE=HelloFromCompose
      - ANOTHER_VARIABLE

Notice that for ANOTHER_VARIABLE, we just listed the variable name. Compose will look for this variable in the environment where docker-compose up is run, including variables loaded from the .env file.

Save the file and exit nano.

Now, stop and restart the Compose application to pick up the changes.

docker-compose down
docker-compose up -d

Get the new container ID using docker ps.

docker ps

Now, use docker exec to check for both environment variables inside the container.

docker exec < container_id > printenv | grep MY_VARIABLE
docker exec < container_id > printenv | grep ANOTHER_VARIABLE

Replace <container_id> with the new Container ID. You should see both MY_VARIABLE=HelloFromCompose and ANOTHER_VARIABLE=ThisIsFromDotEnv in the output.

When publishing, you would typically share the docker-compose.yaml file and provide instructions on how to set the necessary environment variables, either through a .env file or by setting them directly in the environment where Compose is run. Sensitive information should not be hardcoded in the docker-compose.yaml or .env files and should be managed using secrets management solutions.

Summary

In this lab, we learned how to prepare a simple Docker Compose application. This involved installing Docker Compose, verifying its installation, and creating a docker-compose.yaml file to define a basic Nginx web service. We also pulled the necessary Docker image (nginx:latest) in preparation for running the application.

The subsequent steps, which were not fully detailed in the provided content, would likely cover publishing this Compose application to a repository using the docker compose alpha publish command. This would include exploring options like publishing with resolved image digests and including environment variables in the published application definition.