For most users, before you can do anything with umoci, you have to first have an OCI image. At the time of writing, there is no standard way of getting an OCI image. Distribution is still an open topic in the specification, and there are very few implementations of a distribution extension to the OCI specification. I’ve personally worked on one but there is still a lot of work to go before you can skip this step and get OCI images without the need to convert from other things.
In order to get an OCI image, you need to convert it from another container
image format. Luckily, as the OCI spec was based on the Docker image format,
there is no loss of information when converting between the two formats.
skopeo is an incredibly useful tool that allows you to fetch and
convert a Docker image (from a registry, local daemon or even from a file saved
with docker save) to an OCI image (and vice-versa). Read their documentation
for more information about the various other formats they support.
After getting skopeo, you can download an image as follows. Note that you can include multiple Docker images inside the same OCI image (under different “tags”).
% skopeo copy docker://opensuse/amd64:42.2 oci:opensuse:42.2
Getting image source signatures
Copying blob sha256:f65b94255373e4dc9645fcb551756b87726a1c891fe6c89f6bbbc864ff845c15
46.59 MB / 46.59 MB [=========================================================]
Copying config sha256:5af572844af6ae4122721ba6bfa11b4048dc4535a9f52772e809a68cac4e9244
0 B / 805 B [-----------------------------------------------------------------]
Writing manifest to image destination
Storing signatures
% skopeo copy docker://opensuse/amd64:42.1 oci:opensuse:old_42.1
Getting image source signatures
Copying blob sha256:d9e29ed5a74f21e153b05ecc646fe1157fcfa991c9661759986191408665521b
36.60 MB / 36.60 MB [=========================================================]
Copying config sha256:1652ed016d569d50729738e2f4ab3564f7375a25150c4a1ac1cc6687e586a5ce
0 B / 805 B [-----------------------------------------------------------------]
Writing manifest to image destination
Storing signatures
At this point, you have a directory called opensuse which is the downloaded
OCI image stored as a directory. This is currently the only type of layout that
umoci can interact with.