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.
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
