Creating container image without docker

[Bogopo]

Is it possible to build a container image as a simple user account on a system which does not have docker or podman installed? In other words, is there a user-space tool/application which can build an image that could run on a different system which supports OCI containers, such as AKS, etc.

[Mystic]

An image is based on the https://github.com/opencontainers/image-spec/blob/main/spec.md which consists of filesystem layers packaged as tar files, a config json, and a manifest. All of these are referenced with a content addressable digest. If you can create these tar and json files, then you can create an image without a runtime.

There are various tools that do this for specific cases. I think most are shipped as containers and designed to run in CI pipelines, but you can probably get standalone binaries. However, I don't think any of these work with a Dockerfile, so if you need something that supports that syntax, you'll need a container runtime. There are rootless runtimes, but they typically have prerequisites that need to be performed on the host by root in advance, and there's a performance hit from using them.