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.
Instead of creating a simple tarball the tools umoci and skopeo
from the docker tool chain are used to create official docker
images. Along with those tools more information to describe a
container has been added to the schema. A complete container
setup now consists of the following elements
<type image="docker">
<containerconfig name="..." maintainer="..." user="..." workingdir="...">
<entrypoint execute="command">
<argument name="option"/>
...
</entrypoint>
<subcommand execute="command">
<argument name="option"/>
...
</subcommand>
<expose>
<port number="..."/>
...
</expose>
<volumes>
<volume name="..."/>
...
</volumes>
<environment>
<env name="variable" value="value"/>
...
</environment>
<labels>
<label name="..." value="..."/>
...
</labels>
</containerconfig>
</type>
