deb: machine-readable copyright (DEP5) file is incomplete

[thaJeztah]

• Relates to; consider adding machine-readable copyright file (DEP5) file to deb packages docker-ce-packaging#1133
• Relates to; docker/containerd-packaging@ba7f4c5 / Add debian package building for ubuntu xenial containerd-packaging#1

The copyright file (DEP5 https://dep-team.pages.debian.net/deps/dep5/, https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/) currently only lists containerd as upstream, but the containerd.io package also contains runc, which should probably be included as a separate entry.

Currently it uses files: *, which may have to be more granular; https://github.com/docker/containerd-packaging/blob/d6a7e6e2f349710e71a409a6cf2527f48b12e864/debian/copyright#L5-L7

The copyright holder may also have to be updated to match upstream; https://github.com/containerd/containerd/blob/c3efa0cb339ffc2d6c9f03d7fb4e5e6577d0194b/cmd/containerd/main.go#L1-L15

/*
   Copyright The containerd Authors.

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.
*/