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 commits bypasses any URI check if the uri schema is unknown in
RootImport class. This way the URI is bypassed to skopeo if it couldn't
be translated to any known type. That enables referencing images with
any URI supported by skopeo, i.e. DockerHub images.
Kiwi always uses OCI format for container manipulations, so it is
easier to assume the image kept between prepare and create step
is also in OCI format, this way less format convertions are needed.
First, commit refactors RootImport in order to also copy
the imported image after sychronizing the import.
Second, it includes a checksum of the copied image which is
validated in later steps.
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
