Configure docker mirror

docs/how-tos/linux-playgrounds/configure-docker.md

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+# Configuring docker on Linux Playgrounds
+
+## Overview
+
+You have successfully installed Docker on a Linux playground, however you are receiving an error like the following when you try to pull or run any image
+
+> You have reached your pull rate limit You may increase the rate limit by authenticating and upgrading...
+
+This is due to policies introduced by Docker to encourage more people to upgrade their accounts (at a cost).
+
+## Fix
+
+To get round this, KodeKloud have an internal docker repository containing many common images that students will want to work with. Now we must edit some things to make the docker client work with the internal registry.
+
+In Linux playgrounds you should already be logged in as `root`. If not, become root with `sudo -i`
+
+1. Edit hosts file to add an entry for the internal registry.
+
+    ```bash
+    echo "10.0.0.6        docker-registry-mirror.kodekloud.com" >> /etc/hosts
+    ```
+
+1. Create a configuration file for the docker daemon to point to the internal registry
+
+    ```bash
+    mkdir -p /etc/docker
+    cat <<EOF > /etc/docker/daemon.json
+    {
+        "exec-opts": [
+            "native.cgroupdriver=cgroupfs"
+        ],
+        "bip":"172.12.0.1/24",
+        "registry-mirrors": [
+            "http://docker-registry-mirror.kodekloud.com"
+       ]
+    }
+    EOF
+    ```
+
+1. Restart docker
+
+    ```bash
+    systemctl restart docker
+    ```
+
+1. Test
+
+    ```bash
+    docker pull nginx
+    ```
+
+## Installing Docker
+
+### Ubuntu Playground
+
+You should already be logged in as `root`.
+
+Execute the following commands as root
+
+```bash
+apt update
+apt install -y apt-transport-https ca-certificates curl software-properties-common
+curl -fsSL https://download.docker.com/linux/ubuntu/gpg | gpg --dearmor -o /usr/share/keyrings/docker-archive-keyring.gpg
+echo "deb [arch=$(dpkg --print-architecture) signed-by=/usr/share/keyrings/docker-archive-keyring.gpg] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable" >> /etc/apt/sources.list.d/docker.list
+apt update
+apt-cache policy docker-ce
+apt install -y docker-ce
+```
+
+### CentOS playground
+
+CentOS is pre-installed with Red Hat PodMan. To replace this with the real Docker, execute the following commands. You should be logged in as `bob`
+
+```bash
+sudo yum install -y yum-utils
+sudo yum-config-manager --add-repo https://download.docker.com/linux/centos/docker-ce.repo
+sudo yum install docker-ce docker-ce-cli containerd.io docker-buildx-plugin docker-compose-plugin
+sudo yum install -y --allowerasing  docker-ce docker-ce-cli containerd.io docker-buildx-plugin docker-compose-plugin
+sudo chmod a+rw /var/run/docker.sock
+```
+