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Follow the migration of a virtual machine from Docker to Podman.

Linux Containers

It feels like forever since I wrote my Red Hat Enterprise Linux 8 Beta intro to Podman. In fact, it's been quite a while, and a lot has happened since then. For some time now, I've been planning on moving the Digital Ocean Droplet that hosts my sites from the CentOS 7 Docker platform to the CentOS 8 Podman platform. I would really love to do something more sophisticated, but to be honest, a single host running containers is all I need. So, I set out to move from the Docker Compose world to Podman.

In this article, I am going to show you how to use Docker Compose to create a PostgreSQL container and access it using pgAdmin 4, the web-based PostgreSQL admin interface. I will also show you how to access the PostgreSQL database server running in a Docker container from DataGrip IDE. When trying to launch a built container with docker-compose up I'm getting an error. ERROR: for app Cannot start service app: invalid header field value 'oci runtime error: containerlinux.go:247: starting container process caused 'exec: 'script/docker-entrypoint.sh ': stat script/docker-entrypoint.sh: no such file or directory ' ' ERROR: compose.cli.main.main: Encountered errors while.

The latest release of the RHEL 8 / CentOS 8. Red Hat has built its own tools, buildah and podman, which aim to be compatible with existing docker images and work without relying on a daemon, allowing the creation of containers as normal users, without the need of special permissions (with some limitations: e.g. At the moment of writing, it's still not possible to map host ports to the. Well, moving to CentOS 8 meant replacing Docker with Podman. Podman does not have a counterpart to the docker-compose command. Well, it does, sort of. There's a project in the works called podman-compose, which is supposed to do the same basic thing as docker-compose. I wanted to find the 'right' solution, though. Honestly, that was not an easy.

docker-compose alternatives still emerging

I used Docker Compose for one simple reason: I can define my applications in a portable and easy-to-write YAML file, which gives me the ability to group each site with its dependencies. For instance, this site requires a web server running PHP and a database to store its data. Those are two containers, and managing them separately seems silly. Instead, I grouped them in a Docker Compose file. This solution allows me to work with the whole stack when I want to do things like stop services, or pull in updates for the container images I chose to use.

Pods

Well, moving to CentOS 8 meant replacing Docker with Podman. Podman does not have a counterpart to the docker-compose command. Well, it does, sort of. There's a project in the works called podman-compose, which is supposed to do the same basic thing as docker-compose. I wanted to find the 'right' solution, though. Honestly, that was not an easy task. You'd think that a Google search for the 'Podman counterpart to docker-compose ' would get an article about how Podman replaces that functionality with something else. I couldn't find anything, though. What I did find was a reference to pods in Podman. Pods are a way of grouping containers together inside their own namespace, network, and security context. You can even start and stop the whole pod at once. The only thing it doesn't get me is a clean YAML file to define my services.

Note: I found this great example from Red Hat on pods, and it even touches on networking: Podman: Managing pods and containers in a local container runtime.

Podman play

Podman does, however, let you import Kubernetes definitions using the podman play command. Kubernetes definitions are YAML. It sounds like the solution for me. I spent some time trying to learn how to write these things and eventually came across the Kompose tool. Kompose converts docker-compose files into Kubernetes definitions. I thought it was perfect. Except it wasn't. What I needed specifically was a pod definition, and that's not what Kompose gave me. I might have been able to make a pod definition out of what I had, but I had another hunch.

Podman generate

Podman lets you generate Kubernetes definitions from the existing runtime. For example, if you have a running container, you can use podman generate to create a YAML file to define that container. You can also do that with a pod. So, I manually defined one of my WordPress sites in Podman.

Here are a few notes on that process.

Mapping ports

In the Docker world, ports are mapped to containers. That's true in Podman as well—except when you're running inside a pod. See, the pod is like a container of containers. Networking within the pod is more similar to networking within a host OS. Pods reach each other over the local host, and external networking reaches the pod, not the containers directly. When you run containers in a pod, you need to map ports on the pod like you would on the container in Docker or docker-compose. I also found that, although one of the benefits to Podman is the ability to run as a standard user, I had to do all of this as root because of some security problems I ran into when I created the pods. The problems were mainly centered around SELinux. I will likely circle back and try to re-do all of this without superuser privileges.

So, let's create a pod:

This command creates a pod with port 8080 mapped to inside port 80. If you then spin up a container listening on port 80, you'd have connectivity.

Docker Compose Centos Image

Create a container in the pod

Docker Compose Centos 7 Image

To create a container in the pod, use podman run, but don't map a port. This makes more sense when you have more than one container to work with, so I'm going to create a database container and then a WordPress container.

Notice that I pointed the wordpress_db_host in the env: to localhost. That's because the WordPress container is going to find the database container on the local host. Like magic. Our pod has three containers. Yes, I ran two, but the third is the container that does the pod magic.

In my browser, I can get to the WordPress setup page in my container via localhost:8080.

Estimated reading time: 10 minutes

To get started with Docker Engine on CentOS, make sure youmeet the prerequisites, theninstall Docker.

Prerequisites

OS requirements

To install Docker Engine, you need a maintained version of CentOS 7 or 8.Archived versions aren’t supported or tested.

The centos-extras repository must be enabled. This repository is enabled bydefault, but if you have disabled it, you need tore-enable it.

Centos

The overlay2 storage driver is recommended.

Uninstall old versions

Older versions of Docker were called docker or docker-engine. If these areinstalled, uninstall them, along with associated dependencies.

It’s OK if yum reports that none of these packages are installed.

Docker Compose Centos Image

The contents of /var/lib/docker/, including images, containers, volumes, andnetworks, are preserved. The Docker Engine package is now called docker-ce.

Installation methods

You can install Docker Engine in different ways, depending on your needs:

  • Most usersset up Docker’s repositories and installfrom them, for ease of installation and upgrade tasks. This is therecommended approach.

  • Some users download the RPM package andinstall it manually and manageupgrades completely manually. This is useful in situations such as installingDocker on air-gapped systems with no access to the internet.

  • In testing and development environments, some users choose to use automatedconvenience scripts to install Docker.

Install using the repository

Before you install Docker Engine for the first time on a new host machine, you needto set up the Docker repository. Afterward, you can install and update Dockerfrom the repository.

Set up the repository

Install the yum-utils package (which provides the yum-config-managerutility) and set up the stable repository.

Optional: Enable the nightly or test repositories.

These repositories are included in the docker.repo file above but are disabledby default. You can enable them alongside the stable repository. The followingcommand enables the nightly repository.

Docker Compose Centos Image Viewer

To enable the test channel, run the following command:

You can disable the nightly or test repository by running theyum-config-manager command with the --disable flag. To re-enable it, usethe --enable flag. The following command disables the nightly repository.

Docker Compose Centos Imagery

Compose

Learn about nightly and test channels.

Install Docker Engine

  1. Install the latest version of Docker Engine and containerd, or go to the next step to install a specific version:

    If prompted to accept the GPG key, verify that the fingerprint matches060A 61C5 1B55 8A7F 742B 77AA C52F EB6B 621E 9F35, and if so, accept it.

    Got multiple Docker repositories?

    If you have multiple Docker repositories enabled, installingor updating without specifying a version in the yum install oryum update command always installs the highest possible version,which may not be appropriate for your stability needs.

    Docker is installed but not started. The docker group is created, but no users are added to the group.

  2. To install a specific version of Docker Engine, list the available versionsin the repo, then select and install:

    a. List and sort the versions available in your repo. This example sorts results by version number, highest to lowest, and is truncated:

    App store mac catalina. The list returned depends on which repositories are enabled, and is specificto your version of CentOS (indicated by the .el7 suffix in this example).

    b. Install a specific version by its fully qualified package name, which is the package name (docker-ce) plus the version string (2nd column) starting at the first colon (:), up to the first hyphen, separated by a hyphen (-). For example, docker-ce-18.09.1.

    Docker is installed but not started. The docker group is created, but no users are added to the group.

  3. Start Docker.

  4. Verify that Docker Engine is installed correctly by running the hello-worldimage.

    This command downloads a test image and runs it in a container. When thecontainer runs, it prints an informational message and exits.

Docker Engine is installed and running. You need to use sudo to run Dockercommands. Continue to Linux postinstall to allownon-privileged users to run Docker commands and for other optional configurationsteps.

Upgrade Docker Engine

To upgrade Docker Engine, follow the installation instructions,choosing the new version you want to install.

Install from a package

If you cannot use Docker’s repository to install Docker, you can download the.rpm file for your release and install it manually. You need to downloada new file each time you want to upgrade Docker Engine.

  1. Go to https://download.docker.com/linux/centos/and choose your version of CentOS. Then browse to x86_64/stable/Packages/and download the .rpm file for the Docker version you want to install.

    Note: To install a nightly or test (pre-release) package,change the word stable in the above URL to nightly or test.Learn about nightly and test channels.

  2. Install Docker Engine, changing the path below to the path where you downloadedthe Docker package.

    Docker is installed but not started. The docker group is created, but nousers are added to the group.

  3. Start Docker.

  4. Verify that Docker Engine is installed correctly by running the hello-worldimage.

    This command downloads a test image and runs it in a container. When thecontainer runs, it prints an informational message and exits.

Docker Engine is installed and running. You need to use sudo to run Docker commands.Continue to Post-installation steps for Linux to allownon-privileged users to run Docker commands and for other optional configurationsteps.

Upgrade Docker Engine

To upgrade Docker Engine, download the newer package file and repeat theinstallation procedure, using yum -y upgradeinstead of yum -y install, and pointing to the new file.

Docker

Install using the convenience script

Docker Compose Centos Image Usb

Docker provides convenience scripts at get.docker.comand test.docker.com for installing edge andtesting versions of Docker Engine - Community into development environments quickly andnon-interactively. The source code for the scripts is in thedocker-install repository.Using these scripts is not recommended for productionenvironments, and you should understand the potential risks before you usethem:

  • The scripts require root or sudo privileges to run. Therefore,you should carefully examine and audit the scripts before running them.
  • The scripts attempt to detect your Linux distribution and version andconfigure your package management system for you. In addition, the scripts donot allow you to customize any installation parameters. This may lead to anunsupported configuration, either from Docker’s point of view or from your ownorganization’s guidelines and standards.
  • The scripts install all dependencies and recommendations of the packagemanager without asking for confirmation. This may install a large number ofpackages, depending on the current configuration of your host machine.
  • The script does not provide options to specify which version of Docker to install,and installs the latest version that is released in the “edge” channel.
  • Do not use the convenience script if Docker has already been installed on thehost machine using another mechanism.

This example uses the script at get.docker.com toinstall the latest release of Docker Engine - Community on Linux. To install the latesttesting version, use test.docker.com instead. Ineach of the commands below, replace each occurrence of get with test.

Warning:

Always examine scripts downloaded from the internet beforerunning them locally.

If you would like to use Docker as a non-root user, you should now consideradding your user to the “docker” group with something like:

Remember to log out and back in for this to take effect!

Warning:

Adding a user to the “docker” group grants them the ability to run containerswhich can be used to obtain root privileges on the Docker host. Refer toDocker Daemon Attack Surfacefor more information.

Docker Compose Centos Image Download

Docker Engine - Community is installed. It starts automatically on DEB-based distributions. OnRPM-based distributions, you need to start it manually using the appropriatesystemctl or service command. As the message indicates, non-root users can’trun Docker commands by default.

Note:

To install Docker without root privileges, seeRun the Docker daemon as a non-root user (Rootless mode).

Upgrade Docker after using the convenience script

If you installed Docker using the convenience script, you should upgrade Dockerusing your package manager directly. There is no advantage to re-running theconvenience script, and it can cause issues if it attempts to re-addrepositories which have already been added to the host machine.

Uninstall Docker Engine

Docker Compose Centos Image Editor

  1. Uninstall the Docker Engine, CLI, and Containerd packages:

  2. Images, containers, volumes, or customized configuration files on your hostare not automatically removed. To delete all images, containers, andvolumes:

You must delete any edited configuration files manually.

Next steps

  • Continue to Post-installation steps for Linux.
  • Review the topics in Develop with Docker to learn how to build new applications using Docker.
requirements, apt, installation, centos, rpm, install, uninstall, upgrade, update
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