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Aslan Andreev
Aslan Andreev

Docker Book "What Is A Docker Image "


A Docker image has everything needed to run a containerized application, including code, config files, environment variables, libraries and runtimes. When the image is deployed to a Docker environment, it can be executed as a Docker container. The docker run command creates a container from a specific image.




Docker Book "What is a Docker image "



Official images are ones Docker produces, while community images are images Docker users create. CoScale agent is an official Docker image that monitors Docker applications. Datadog/docker-dd-agent, a Docker container for agents in the Datadog log management program, is an example of a community Docker image.


Users can also create new images from existing ones and use the docker push command to upload custom images to the Docker Hub. To ensure the quality of community images, Docker provides feedback to authors prior to publishing. Once the image is published, the author is responsible for updates. Authors must be cautious when sourcing an image from another party because attackers can gain access to a system through copycat images designed to trick a user into thinking they are from a trusted source.


With this method, users run a container from an existing Docker image and manually make any needed changes to the environment before saving the image. The interactive method is the easiest way to create docker images. The first step is to launch Docker and open a terminal session. Then use the Docker run command image_name:tag_name. This starts a shell session with the container that was launched from the image. If the tag name is omitted, Docker uses the most recent version of the image. After this, the image should appear listed in results.


This approach requires making a plain text Dockerfile. The Dockerfile makes the specifications for creating an image. This process is more difficult and time-consuming, but it does well in continuous delivery environments. The method includes creating the Dockerfile and adding the commands needed for the image. Once the Dockerfile is started, the user sets up a .dockerignore file to exclude any files not needed for the final build. The .dockerignore file is in the root directory. Next, the Docker build command is used to create a Docker image and an image name and tag are set. Lastly, the Docker images command is used to see the created image.


Now, open up Ubuntu or whatever distribution you've installed from Microsoft Store. Execute the docker --version and docker-compose --version commands to make sure that the installation was successful.


This concept will become a lot clearer in upcoming sections of this book. But for now, just keep in mind that images are multi-layered read-only files carrying your application in a desired state inside them.


Okay, enough talking. Now it's time for you to understand how all these pieces of the puzzle you just learned about work in harmony. Before I dive into the explanation of what really happens when you run the docker run hello-world command, let me show you a little diagram I've made:


The CONTAINER ID is 9f21cb777058 which is the first 12 characters of the full container ID. The full container ID is 9f21cb77705810797c4b847dbd330d9c732ffddba14fb435470567a7a3f46cdc which is 64 characters long. This full container ID was printed as the output of the docker container run command in the previous section.


I hope that you remember the container you started in the previous section. It's still running in the background. Get the identifier for that container using docker container ls (I'll be using hello-dock-container container for this demo). Now execute the following command to stop the container:


As an example, if you run a container using the ubuntu image by executing docker container run ubuntu you'll see nothing happens. But if you execute the same command with the -it option, you should land directly on bash inside the Ubuntu container.


So in the end the docker container run --rm -v $(pwd):/zone fhsinchy/rmbyext pdf command translates to rmbyext pdf inside the container. Executable images are not that common in the wild but can be very useful in certain cases.


Take the official mysql image, for example. If you want to run a container using a specific version of MySQL, like 5.7, you can execute docker container run mysql:5.7 where mysql is the image repository and 5.7 is the tag.


From the very beginning of this book, I've been saying that images are multi-layered files. In this sub-section I'll demonstrate the various layers of an image and how they play an important role in the build process of that image.


Well, Docker has a similar concept. The .dockerignore file contains a list of files and directories to be excluded from image builds. You can find a pre-created .dockerignore file in the hello-dock directory.


This .dockerignore file has to be in the build context. Files and directories mentioned here will be ignored by the COPY instruction. But if you do a bind mount, the .dockerignore file will have no effect. I've added .dockerignore files where necessary in the project repository.


Before you execute the command, though, make sure you've opened your terminal in the same directory where the docker-compose.yaml file is. This is very important for every docker-compose command you execute.


The --detach or -d option here functions the same as the one you've seen before. The --file or -f option is only needed if the YAML file is not named docker-compose.yaml (but I've used here for demonstration purposes).


Let's start writing the docker-compose.yaml file. Apart from the api and db services there will be the client and nginx services. There will also be some network definitions that I'll get into shortly.


Try adding and deleting notes to see if the application works properly. The project also comes with shell scripts and a Makefile. Explore them to see how you can run this project without the help of docker-compose like you did in the previous section.


By default, the docker daemon listens to the docker.sock UNIX socket. If you have any use case to access the docker API remotely, you need to expose it over a host port. One such use case is running Docker as Jenkins agents.


For example, if you can use a base image of Ubuntu and create another image with the Nginx application. A base image can be a parent image or an image built from a parent image. Check out his docker article to know more about it.


Tip: When using docker in enterprise networks/project, set up your own docker registries instead of using the public docker hub. All cloud providers have their own container registry services.


Docker engine is composed of the docker daemon, rest interface, and the docker CLI. Docker daemon is the systemd dockerd service responsible for building the docker images and sending docker instructions to containerd runtime.


Just as source code, Docker images are likely to be published in shared repositories, publicly on hub.docker.com, or in a company registry and therefore accessible to users who are not allowed to access the contained secrets.


More surprisingly, we did not find as many secrets as in public source code: the ratio is close to 1 secret for every 500 files scanned in Docker images, half of what we usually find in public source code. In our opinion, this is because users pushing docker images are more concerned about security than developers in general.


Docker images, because of their structure and usage, are likely to contain hidden secrets. As our research shows, there are fewer secrets hidden in public Docker images than in source code on GitHub but still 7% of the public images contain secrets. This remains a major security issue for companies willing to ensure proper secret management. While good development practices can reduce the margin of error when building the images, automatic scanning can avoid many leaks. This is why GitGuardian recently released a Docker image scanner that can be used in CI to ensure that Docker images are free from secrets. You can find more information on how to scan your docker images using gg-shield in this documentation.


That's why we suggest you use the json-file driver and blocking mode with a dedicated logging container to get the most of your log management setup. Luckily it's the default log driver setup, so you don't need to configure anything in the /etc/docker/daemon.json file.


The log file format for the json-file logging driver is machine readable JSON format with a timestamp, stream name and the log message. Therefore users prefer the docker logs command to see the logs on their console.


Docker has a dedicated command which lists container logs. The docker logs command. The flow will usually involve you checking your running containers with docker ps, then check the logs by using a container's ID.


This command is building a Docker image from the docker file **myapp **which is present in the current directory. It has multiple layers stacked on top of each other and is represented as a single object. A docker image is created using a docker file, we will get to that in a bit.


You can also see all the images using the docker image command. You can further see Docker Mastery: with Kubernetes +Swarm from a Docker Captain course to learn more about essential Docker concepts and commands.


Docker Container is a running instance of your docker image. When you run your docker image using the $ docker run command then these containers are created. Y*ou can run as many containers as you want using the same docker image.*


Now the question comes, how can programmers use Docker? Well, to use Docker you need to install it. Once you install Docker, you can execute docker commands to create a Docker file that contains the instructor to create a Docker image.


The following snippet shows the configuration file docker/nginx/default.conf which tells NGINX to serve static files from /var/www/html and forward PHP requests to PHP-FPM. You do not need to make any changes to the NGINX configuration when dockerizing your web application. 041b061a72


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