Docker - Import from LXC image (tar.zst)

1. Get an LXC image:

http://download.proxmox.com/images/system/

For my use case, I downloaded Debian 11.

2. Extract the tar.zst archive

mkdir archive
tar --use-compress-program=unzstd -xvf archive-filename.tar.zst -C archive

3. Create the Docker image

tar -C ./archive -c . | sudo docker import - project/image

4. Create a container from the image

docker run -it project/image /bin/bash

---

mount the container volume

pct mount 115

cp to /tmp (archiving doesn’t work well in the source folder, for some reason)

cp -rp /var/lib/lxc/115/rootfs /tmp/rootfs 
pct unmount 115

tar.gz the folder

tar -czvf rootfs.tar.gz /tmp/rootfs 
rm -r /tmp/rootfs

scp it to docker server

scp /tmp/rootfs.tar.gz tomm@10.10.10.5:/home/tomm 
rm /tmp/rootfs.tar.gz

untar the archive

tar -xzf rootfs.tar.gz 
rm rootfs.tar.gz

import the rootfs folder as a docker image

sudo tar -C ./rootfs -c . | sudo docker import - debian/infralm
sudo rm -r rootfs

provide a docker-compose.yml

version: '3'
 
services:
  debian:
    image: debian/infralm
    container_name: infralm
    command: tail -f /dev/null
    ports:
      - "8000:80"
      - "2000:20"
      - "4430:443"

create a container with that docker-compose

sudo docker-compose up -d

NOTE

The command: tail -f /dev/null is often used as a workaround to keep the container running indefinitely.

When you start a container without specifying a command, Docker will execute the default command defined in the Dockerfile. In some cases, this default command might not keep the container running, leading Docker to exit the container immediately after starting it. By specifying tail -f /dev/null as the command, you ensure that the container stays running indefinitely, allowing you to interact with it or attach to its shell.

Enter this container

docker exec -it infralm bash

Ansible playbook:

---
- name: Transfer rootfs from Proxmox to Docker server
  hosts: proxmox
  become: true
  tasks:
    - name: Mount container volume
      shell: pct mount 115
 
    - name: Copy rootfs to temporary directory
      shell: cp -rp /var/lib/lxc/115/rootfs /tmp/rootfs
 
    - name: Unmount container volume
      shell: pct unmount 115
 
    - name: Create tar.gz archive
      shell: tar -czvf rootfs.tar.gz -C /tmp/ rootfs/
      args:
        chdir: /tmp
 
    - name: Transfer archive to Docker server
      shell: scp /tmp/rootfs.tar.gz tomm@10.10.10.5:/home/tomm/rootfs.tar.gz
 
    - name: Remove temporary files
      file:
        path: /tmp/rootfs.tar.gz
        state: absent
      register: remove_tmp_files
 
- name: Import rootfs as Docker image and run container
  hosts: docker_server
  become: true
  tasks:
    - name: Untar archive
      unarchive:
        src: /home/tomm/rootfs.tar.gz
        dest: /home/tomm
        remote_src: yes
 
    - name: Import rootfs folder as Docker image
      shell: tar -C /home/tomm/rootfs -c . | sudo docker import - debian/infralm
 
    - name: Delete extracted archive
      shell: rm -r /home/tomm/rootfs
 
    - name: Provide docker-compose.yml
      template:
        src: docker-compose.yml.j2
        dest: /home/tomm/docker-compose.yml
 
    - name: Create container with docker-compose
      shell: docker-compose up -d
      args:
        chdir: /home/tomm