Host OS: macOS but this is likely irrelevant, I can't test on other host OS.Containers 101: attach vs.I cannot untar an archive as root on xenial due to Cannot mkdir: Permission denied.□ containerd - a secret hero of the Cloud Native world.Why and How to Use containerd From Command Line.Learning Docker with Docker - Toying With DinD For Fun And Profit.Cracking the Docker CLI: How to Grasp Container Management Commands.How to Run a Container Without an Image.However, this might change soon, thanks to the ongoing attempt to offload more and more lower-level tasks from Docker to containerd. It would be great if it were possible to use ctr to inspect images owned by dockerd, but a quick check ( ctr -namespace moby image ls) showed that it's not the case yet. The above trick is what I used before this recent realization of how to use docker export in combination with just created container. $ ctr image mount docker.io/library/nginx:latest rootfs The cool part about containerd is that it provides a much more fine-grained control over the typical container management tasks than Docker does: $ ctr image pull docker.io/library/nginx:latest And containerd often comes with its own command-line client, ctr, that can be used, in particular, to inspect images. It means that if you have a dockerd daemon running on a machine, most likely there is a containerd daemon somewhere nearby as well. Bonus: Mount container images as host foldersĪs you probably know, Docker delegates more and more some of its container management tasks to another lower-level daemon called containerd. ⚠️ Caveat: I couldn't find a way to preserve the file ownership information with docker build -o. Thanks to Chris Guest for pointing me to this amazing feature! ĭrwxr-xr-x 2 vagrant vagrant 4096 Aug 22 00:00 binĭrwxr-xr-x 2 vagrant vagrant 4096 Jun 30 21:35 bootĭrwxr-xr-x 4 vagrant vagrant 4096 Sep 12 14:07 devĭrwxr-xr-x 2 vagrant vagrant 4096 Aug 23 03:59 docker-entrypoint.dĭrwxr-xr-x 2 vagrant vagrant 4096 Aug 23 03:59 tmpĭrwxr-xr-x 11 vagrant vagrant 4096 Aug 22 00:00 usrĭrwxr-xr-x 11 vagrant vagrant 4096 Aug 22 00:00 var $ DOCKER_BUILDKIT=1 docker build -o rootfs. The -output flag works only if BuildKit is used, so you'll either need to go with docker buildx build or set the DOCKER_BUILDKIT=1 variable: $ echo 'FROM nginx' > Dockerfile Generally, building an image triggers (temporary) container runs, but a Dockerfile without any RUN instructions should do the trick! Turns out that starting from Docker 18.09 (released ~early 2019), it's possible to specify a custom location for the docker build command results using the -output flag. ![]() Don't forget to docker rm the temporary container □ docker build can also be helpful ![]() $ docker export $ -o Īnd a handy oneliner (assuming the image has already been pulled and the target folder created): $ docker export $(docker create nginx) | tar -xC The problem with this command is that it expects a container and not an image name: $ docker export nginx -o Įrror response from daemon: No such container: nginxĪn obvious solution would be to run the container and repeat the export attempt: $ docker pull nginx Let's try our luck with it: $ docker export -helpĮxport a container's filesystem as a tar archive The second command that looks relevant is docker export. docker export is what you need (but with a trick) The docker save command, also known as docker image save, dumps the content of an image in its canonical layered representation while we're interested in the final state of the filesystem that image would produce when mounted.
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