This also incorporates the version string into the package name so
that incompatibile versions will fail to connect.
Arbitrary choices:
- The proto3 package name is runtime.v1alpha2. The proto compiler
normally translates this to a go package of "runtime_v1alpha2", but
I renamed it to "v1alpha2" for consistency with existing packages.
- kubelet/apis/cri is used as "internalapi". I left it alone and put the
public "runtimeapi" in kubelet/apis/cri/runtime.
This change moves the code specific to docker to kubelet/dockertools,
while leaving the common utility functions at its current package
(pkg/securitycontext).
When we deprecate dockertools in the future, the code will be moved to
pkg/kubelet/dockershim instead.
