Docker Import

Introduction to Docker Import

Docker import is a Docker command to create a Docker image by importing the content from an archive or tarball which is created by exporting a container. We can specify URL or ‘-‘ to import data or content from the archive. The URL can point to a location where archive file is present and ‘-‘ (dash) is used to import data directly from the STDIN i.e. standard input. We can apply Dockerfile instructions while creating the image using Docker import however there are limited Dockerfile instructions that are supported over here.

Syntax: 

docker import [options] file|URL|- [REPOSITORY[:TAG]]

Options:

1. -c, –change: It is used to provide Dockerfile instructions if needed while creating the Docker image. It takes input as a list.
2. -m, –message: It is used to add a message to the Docker image while importing from an archive file. It is a string.

Here is the snapshot of the ‘docker import –help’ output that tells what we just discussed above.

How to Import Docker Image?

In order to import a Docker image, first, we must have an exported archive file of a container. So when we export any container, it actually exports containers as a regular Linux file system in an archive file; then we have to import this archive file as a Docker image and when we run any container using this new imported Docker image then it works in the same way as it was working in the old container. This actually saves our time to build the Docker image from scratch if there is any changes are required. Docker import only supports these Dockerfile instructions: CMD, ENTRYPOINT, ENV, EXPOSE, ONBUILD, USER, VOLUME, WORKDIR.

Examples

Let’s understand the process using an example:

Example #1 – Exporting a Container

1. First, create a Docker image using below Dockerfile:

docker build -t my-image:v2 .

2. Run this image as a container using the below command: –

docker run my-image:v2
docker ps –a

3. Export the container in a tar file named helloworld.tar using the below command: –

Syntax: 

docker export <container_ID> > <file_name>
docker export <container_name> > <file_name>

Example: 

docker export 3673f8996e1a > helloworld.tar

Explanation: In the above snapshot, we have created a Docker image named ‘my-image:v2’ and started a container using it, and the container just displayed output “Hello World!!!” that is mentioned in the Docker image and it gets stopped. Then, we have exported this container as a tar file named “helloworld.tar”.

Example #2 – Investigating the Exported Archive File

4. Now create a folder ‘helloworld’ and extract the content of the tar file to this folder as below: –

mkdir helloworld
tar -xf helloworld.tar -C helloworld

5. And if we check the folder structure of helloworld, it looks like a regular Linux filesystem.

tree –L 1 helloworld

Example #3 – Different Ways to Import the Docker Image

6.Let’s import this tar file as a Docker image using the below command: –

Syntax: 

docker import <archive_name> <Image_name>
cat <archive_name> | docker import - <image_name>

Example: 

docker import helloworld.tar my-imported-img:v1
cat helloworld.tar | docker import – my-imported-img:v2

docker import history my-imported-img:v1

Explanation: In the above snapshot, we can see there is no difference in the images so we can use any of the above commands to import the Docker image from an archive.

7. We can also pass comment while importing the Docker image using the ‘-m’ option as below: –

Syntax:

cat <archive_name> | docker import -m <comment or message> - <image_name>

Example:

cat helloworld.tar | docker import -m “New Image imported” – my-imported-image:v3

Example #4 – Verify the Imported Image

8. Run a container using this new Docker image to verify if it is working as the earlier container:

docker run --it my-imported-img:v1 sh

Explanation: – In the above example, we can see that we need to specify the command while running a container using the imported Docker image as there is no command to run so we get an error if we run the command

docker run my-imported-img:v1

Example #5 – Adding Dockerfile instruction while importing the Image

9. We can apply Dockerfile instruction while importing the Docker image using the ‘-c’ option as below: –

Syntax:

docker -c <Dockerfile Instruction> <archive_file> <image_name>

Example:

docker -c ‘CMD echo “HelloWOrld!!!”’ helloworld.tar my-imported-img:v4

10. Now, let’s say we have to add a file to the previous image. We can do this in two ways, the first one is to edit the Dockerfile and rebuild the image or run a container using this image and create the file and export the container and then import it as a new Docker image. The easy way is to run a container and make the required changes and export it and import it as a Docker image.

docker run -it my-image:v1 sh

Explanation: In the above example, created a new container using the ‘my-image:v1’ and connected to it; created a testfile.txt and populated some data in it. Exported that container and imported as a Docker image named ‘my-imported-img:v5’ and ran a new container using this newly imported image and when we can see that the ‘testfile.txt’ exists in this container as well.

Advantages

1. It is very useful if we have to make minor changes to the existing image but do not want to rebuild the Docker image as it takes much time to rebuild from scratch again. It saves our time by avoiding the rebuild process of the image as well.
2. We can share the Docker images with others without using the Docker registry as we can export it as a tar file and share the tar as we share regular files and one can import it using Docker import.
3. It is also useful to create a single layer Docker image file that is sometimes required to improve the performance, etc. , however, it is not recommended by Docker.

docker image history my-image:v1
docker image history my-imported-img:v5

Explanation: In the above snapshot, we can see that the ‘my-image:v1’ image has 3 layers however the imported image ‘my-imported-img:v5’ has only one layer.

Conclusion

Docker Import is a great tool that helps us to save the time by avoiding the re-build process of Docker image for minor changes and also to share the Docker image with others without Docker registry, however, we have ‘load’ and ‘save’ command to backup or share the Docker image. It only works with the container. We mostly use this to improve the performance of the container as it creates a single layer Docker image.

Recommended Articles

This is a guide to Docker Import. Here we discuss the Introduction, How to Import Docker Image with examples? respectively. You may also have a look at the following articles to learn more –

1. Docker Hosts
2. Docker Stop Container
3. Docker Export
4. Docker Volume