Docker is a popular platform used for building, shipping, and running applications inside containers. One common task when working with Docker is transferring images from one host to another. While Docker Hub and other repositories are typical methods for achieving this, sometimes you might prefer or need to avoid using a repository. This article explores how to copy Docker images between hosts without relying on a central repository.

Techniques for Copying Docker Images

1. Using Docker Save and Load Commands

The docker save and docker load commands are versatile and enable the transfer of Docker images in the form of tarballs:

Step-by-Step Process

1. Save the Docker Image: First, you need to convert the Docker image into a tarball file on the source host.

   docker save -o <path_to_tarball> <image_name>:<tag>

Example:

   docker save -o myimage.tar myimage:latest

1. Transfer the Tarball: Next, transfer the tarball to the destination host. You can use tools like scp or rsync for this purpose.

   scp myimage.tar user@destination_host:/path/to/destination

1. Load the Docker Image: Once the tarball is on the destination host, you can load it back into Docker.

   docker load -i <path_to_tarball>

Example:

   docker load -i myimage.tar

Advantages and Considerations

• Portability: Tarballs can be moved without Docker being directly involved in the transfer.
• Security: The transfer tools enable the use of secure channels.
• Dependencies: You avoid overhead associated with network repositories.

2. Using Docker Export and Import

For running containers, docker export and docker import can be utilized:

Differences from Save/Load

• Scope: docker export works with containers, not images. Thus, metadata about volumes or the base image layer is not preserved.
• Practical Use: Ideal for scenarios where you want to capture the filesystem at a specific point in time.

Steps Involved

1. Export the container:

   docker export -o <path_to_tarball> <container_id_or_name>

Example:

   docker export -o mycontainer.tar mycontainer

1. Transfer the exported tarball:

   scp mycontainer.tar user@destination_host:/path/to/destination

1. Import to create a new image:

   cat <path_to_tarball> | docker import - <new_image_name>:<tag>

Example:

   cat mycontainer.tar | docker import - mynewimage:latest

Key Considerations

• Layer Preservation: export/import does not preserve layers or Dockerfile instructions. It's more akin to a snapshot of the final state.
• Use Case: Typically used when the history of the image isn't crucial, or when working with container state rather than image state.

Additional Tips and Tools

Using Compression

Compressing the image tarball with tools like gzip can save bandwidth:

• Compression:

  docker save <image_name> | gzip > <image>.tar.gz

• Decompression on the target host before loading:

  gunzip -c <image>.tar.gz | docker load

Network Tools

• nc (netcat): For direct network transfer, bypassing files:
  On the source host:

  docker save <image_name> | nc <destination_host> <port>

On the destination host:

  nc -l -p <port> | docker load

Ensure firewalls or network policies allow the specified port.

Conclusion

Copying Docker images without a repository can be efficiently handled using Docker's built-in commands. This method proves advantageous in scenarios where network accessibility or registry use is limited. By understanding these techniques, you can deploy, manage, and maintain your Docker images across various environments without dependency on external repositories.

Summary Table

By utilizing these methods, Docker users retain control, flexibility, and portability in their containerization workflows, ensuring smooth transitions and deployments across different environments.