Automatic merge from submit-queue
Base etcd-empty-dir-cleanup on busybox, run as nobody, and update to etcdctl 3.0.14
**What this PR does / why we need it**: since the `etcd-empty-dir-cleanup` image just uses a simple shell script and `etcdctl`, we can base it on busybox, which is a smaller target than alpine.
I've also updated this to use an `etcdctl` from etcd 3.0.14, which matches the version of etcd we're running in 1.6 clusters (I believe), and changed the tag to match the `etcdctl` version.
Tested in my own e2e cluster, where it seems to work.
I haven't pushed the image yet, so e2e tests *may* fail. Tagging `do-not-merge`; if you think this looks good, I'll push the image and retest.
**Release note**:
```release-note
```
cc @timstclair @mml @wojtek-t
84b74074a4