Docker image format
I would like to build a Docker image without docker iself. I have looked at Packer, but it requiers to have Docker installed on the builder host.
I have looked at the Docker Registry API documentation but this information doesn't appear to be there.
I guess that the image is simply a tarball, but I would like to see a complete specification of the format, i.e. what exect format is required and whethere there are any metadata files required. I could attempt downloading an image from the registry and look what's inside, but there is no information on how to fetch the image itself.
The idea of my project is to implement a script that creates an image from atefacts I have compiled and uploads it to the registry. I would like to use OpenEmbedded for this purpose, essentially this would be an extention to Bitbake.
Answer
The Docker image format is specified here: https://github.com/docker/docker/blob/master/image/spec/v1.md
The simplest possible image is a tar file containing the following:
repositories
uniqid/VERSION
uniqid/json
uniqid/layer.tar
Where VERSION contains 1.0, layer.tar contains the chroot contents and json/repositories are JSON files as specified in the spec above.
The resulting tar can be loaded into docker via docker load < image.tar