This commit refactors the OCI images support:
* added import_container_image and export_container_image methods
to oci_tools classes. 'umoci' and 'buildah' consume different
formats thus the inital skopeo call to import a container is tool
dependent.
* use oci-archive transport for packing the OCI images, this causes
docker and oci operations to just diverge on transport type.
* add_tag method no longer needed in oci_tools/base, skopeo is used
for that matter.
* container/docker.py class is no longer needed. The difference
between docker and OCI images is just on packing format which is just
a parameter in skopeo. It does not deserve a dedicated class
* system/root_import/docker.py class no longer needed. The difference
between OCI and Docker class was just the transport type for the
skopeo call. It does not deserve a dedicated class
This commit adds support for OCI images. Most of the docker related
code is reused for OCI classes and Docker classes have been refactored
so now they are a splecialization of the OCI classes. It is done this
way since KIWI internally only uses OCI format to operate with
containers, therefore docker images just differ from OCI images by
the way they are packaged or unpackaged.
This commit includes the root import feature. A `derived_from`
attribute has been included with the <type> section to make
reference to the image file to import. The image format to import
is assumed to be the same as the build type to import.
The current format supported is docker
