We can save Docker images to local *.tar files, and later load and run those Docker images from local *.tar files. I’m documenting my learning experimentations in this post.

032-feature-image.png
Python: Docker image build – save to and load from *.tar files.

I'm aware that we can create a local registry and push Docker images into this registry instead of the public ones. I like local files, I'm exploring this option first.

The code used in this post is the exact code developed for Python: Docker image build -- “the Werkzeug” problem 🤖! It can be cloned with: 

git clone -b v1.0.2 https://github.com/behai-nguyen/flask-restx-demo.git

Please note, in this post, all Docker images are built on Windows 10 Pro.

Table of contents

Environments

  1. Synology DS218 -- DSM 7.1-42661 Update 3.
  2. Windows 10 Pro -- version 10.0.19044 build 19044.
  3. Windows “docker” CLI -- version 20.10.12, build e91ed57.
  4. Windows Docker Desktop -- version 4.4.3. The latest version is 4.10.1.
  5. Synology DS218 -- it's accessed via its device name omphalos-nas-01 instead of its IP address.

Build and Save Windows images

Please see also this official document docker save.

To make it clean, I also removed all related containers and images before testing.

❶ Build as normal: 

F:\flask_restx_demo\>docker build --tag flask-restx-demo .

We can save images by using standard output redirection > or by the --output option. Images can be identified by their names, i.e. the values in the REPOSITORY column when listing images, or by their Ids.

❷ Save by image name: 

F:\flask_restx_demo\>docker save flask-restx-demo > E:\docker-images\flask_restx_demo-win-by-name.tar

❸ Save by image Id:

Images' Ids can be obtained via command: 

docker images

Saving the same image using its Id and --output</span> option: 

F:\flask_restx_demo\>docker save 1bbe6c6752a2 --output E:\docker-images\flask_restx_demo-win-by-id.tar

The docker save command does not seem to print out anything:

031-01-windows-windows-save-to-files-a.png

The two ( 2 ) output files and their sizes. Please note the slight difference in sizes:

031-01-windows-windows-save-to-files-b.png

Build and Save linux/arm64 images

We've previously discussed how to prepare for and to build multi-arch images on Windows 10 Pro. Please see Synology DS218: unsupported Docker installation and usage... | Test installation -- points ❶ and ❸.

Basically, to prepare multi-arch build profile mybuilder, run: 

C:\>docker buildx create --name mybuilder --driver-opt network=host --use

Then to build, and to push to https://hub.docker.com/ after finishing building: 

F:\some_project>docker buildx build --platform linux/arm64 --tag username/some-project --push .

❶ Try building an image for linux/arm64 only, that is, do not push the image onto the registry afterward: 

F:\flask_restx_demo>docker buildx build --platform linux/arm64 --tag flask-restx-demo-arm64 .

No image loaded: this should make sense, since this build is for linux/arm64 not Windows.

While research for the problem, I found this post Where did the built multi-platform image go? #166, answers by user barcus provide the solution.

❷ To build and to save locally: 

F:\flask_restx_demo>docker buildx build --platform linux/arm64 --output "type=docker,push=false,name=flask-restx-demo-arm64,dest=E:\docker-images\flask-restx-demo-arm64.tar" .

Image file E:\docker-images\flask-restx-demo-arm64.tar, its size is less than half of the previous two ( 2 ):

031-02-windows-linux-arm64-save-to-files.png

Load and run Windows images

Please see also this official document docker load.

The two image files flask_restx_demo-win-by-id.tar and flask_restx_demo-win-by-name.tar should be the same. I'm testing them both anyway.

flask_restx_demo-win-by-id.tar

❶ Start clean. Remove any existing related containers and images.

❷ To load an image file, use: 

F:\>docker load --input E:\docker-images\flask_restx_demo-win-by-id.tar

The loaded image has the same Id with the original image's: 1bbe6c6752a2 -- please see the screen capture below:

031-03-load-win-image-by-id-on-windows.png

Since the loaded image does not have an image name assigned, we have to run it via its image Id.

❸ Run the loaded image by its Id: 1bbe6c6752a2 

F:\>docker run --publish 8000:8000 --rm 1bbe6c6752a2

We run it in none detached mode, that is, without option -d, so that we can see the output on command line screen:

031-04-run-win-image-by-id-on-windows.png

❹ We can access the container via the following URLs:

Swagger UI URL:

http://localhost:8000/api/v1/ui

API URL:

http://localhost:8000/api/v1/trees

flask_restx_demo-win-by-name.tar

❶ Start clean. Stop and remove previous test container.

❷ Remove any existing related images.

❸ To load: 

F:\>docker load --input E:\docker-images\flask_restx_demo-win-by-name.tar

This time, the loaded image has both REPOSITORY and TAG properties defined:

031-05-load-win-image-by-tag-on-windows.png

❹ Run the loaded image with a different port to the previous run --publish 9010:8000: 

F:\>docker run --publish 9010:8000 --rm flask-restx-demo

❺ As before, we can access the container via the following URLs, please note the port: 

http://localhost:9010/api/v1/ui
http://localhost:9010/api/v1/trees

Load and run linux/arm64 image on Synology DS218

Recall from section Build and Save linux/arm64 images, the image file has been saved to E:\docker-images\flask-restx-demo-arm64.tar.

❶ On the Synology DS218 box, start the Docker daemon if not already started: 

$ sudo dockerd &

❷ Start clean. On the Synology DS218 box, remove all related containers and images.

❸ Copy E:\docker-images\flask-restx-demo-arm64.tar to Synology DS218 box's $HOME/Test/.

❹ Load the image file with: 

behai@omphalos-nas-01:~/Test$ sudo docker load --input flask-restx-demo-arm64.tar

The image loads successfully, please see verifications in the screen capture below:

031-06-load-linux-arm64-on-ds218.png

❺ Run the loaded image with: 

behai@omphalos-nas-01:~/Test$ sudo docker run --network=host -v "/run/docker.sock:/var/run/docker.sock" --rm flask-restx-demo-arm64

It runs successfully:

031-07-run-linux-arm64-on-ds218.png

❻ As before, we can access the container via the following URLs from Windows 10 Pro: 

http://omphalos-nas-01:8000/api/v1/ui
http://omphalos-nas-01:8000/api/v1/trees

Please note, the repo now includes a test HTML page using JQuery test_client_app\jquery-ajax\TreeAPIClient.html, just copy it to a web site or a virtual web directory, and run it from there, for API URL use http://omphalos-nas-01:8000/api/v1/trees.

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I do enjoy learning and blogging about this feature of Docker, I find this feature very useful... Thank you for reading, and I hope you find the info in this post useful also.