Use the sinit conventions for init

This should make it easier to switch out `init` for other versions,
although the `getty` config still needs to be removed.

- use `/bin/rc.init` for start
- use `rc.shutdown` for stop
- make `rc.init` run other code.
- merge `containers` and `containerd` startup code

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

pkg/init/Dockerfile

@@ -12,3 +12,4 @@ WORKDIR /
 COPY --from=mirror /out/ /
 COPY init /
 COPY etc etc/
+COPY bin bin/