Add plugin to generate docker (OCI) images

Open Container Initiative (OCI) compliant images spec is here:

• https://github.com/opencontainers/image-spec

No need to reimplement the wheel here, the plugin can probably call any of the utilities that can generate OCI images

• docker
• buildah: https://github.com/projectatomic/buildah
• oci-image-tools: https://github.com/opencontainers/image-tools