Add findutils to kubelet container.

kubelet wants a fully featured `find` now it seems:
```
E0130 14:50:23.475987     654 fsHandler.go:121] failed to collect filesystem stats - rootDiskErr: <nil>, rootInodeErr: cmd [find /var/lib/docker/overlay2/2c4a7692faaaa08afc65430415958e14ae64cd419ad4baa8555cddda3c934814/diff -
xdev -printf .] failed. stderr: find: unrecognized: -printf
BusyBox v1.27.2 (2017-12-12 10:41:50 GMT) multi-call binary.

Usage: find [-HL] [PATH]... [OPTIONS] [ACTIONS]

Search for files and perform actions on them.
```

Unsure when this started.

Signed-off-by: Ian Campbell <ijc@docker.com>

Author: Ian Campbell

pkg/kubelet/Dockerfile

@@ -86,6 +86,7 @@ RUN apk add --no-cache --initdb -p /out \
     curl \
     ebtables \
     ethtool \
+    findutils \
     iproute2 \
     iptables \
     musl \