fix RUNC.md vs vendor.conf mismatch
Actually we have been testing containerd with the runc version that is defined
in vendor.conf rather than the one defined in RUNC.md. (`script/setup/install-runc`).
This commit makes sure that the revision defined in vendor.conf is always the desired one.
containerd is built with OCI support and with support for advanced features provided by [runc](https://github.com/opencontainers/runc).
-We depend on a specific `runc` version when dealing with advanced features. You should have a specific runc build for development. The current supported runc commit is:
-
-RUNC_COMMIT = a618ab5a0186905949ee463dbb762c3d23e12a80
+We depend on a specific `runc` version when dealing with advanced features. You should have a specific runc build for development. The current supported runc commit is described in [`vendor.conf`](vendor.conf). Please refer to the line that starts with `github.com/opencontainers/runc`.
For more information on how to clone and build runc see the runc Building [documentation](https://github.com/opencontainers/runc#building).
