move dependencies to vendor

vendor/github.com/docker/libcontainer/ROADMAP.md

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+# libcontainer: what's next?
+
+This document is a high-level overview of where we want to take libcontainer next.
+It is a curated selection of planned improvements which are either important, difficult, or both.
+
+For a more complete view of planned and requested improvements, see [the Github issues](https://github.com/docker/libcontainer/issues).
+
+To suggest changes to the roadmap, including additions, please write the change as if it were already in effect, and make a pull request.
+
+## Broader kernel support
+
+Our goal is to make libcontainer run everywhere, but currently libcontainer requires Linux version 3.8 or higher. If you’re deploying new machines for the purpose of running libcontainer, this is a fairly easy requirement to meet. However, if you’re adding libcontainer to an existing deployment, you may not have the flexibility to update and patch the kernel.
+
+## Cross-architecture support
+
+Our goal is to make libcontainer run everywhere. Recently libcontainer has
+expanded from its initial support for x86_64 systems to include POWER (ppc64
+little and big endian variants), IBM System z (s390x 64-bit), and ARM. We plan
+to continue expanding architecture support such that libcontainer containers
+can be created and used on more architectures.