Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Drop capabilities of CoreDNS container and run in read-only

**What this PR does / why we need it**: Make the CoreDNS container more secure by dropping (root) capabilities. Improve the integrity of the of the container by running the whole container in read-only.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #

**Special notes for your reviewer**:
Same [changes](https://github.com/coredns/deployment/commit/aba0245609821e8bba8b4ecd6a32c1a02a7bf658) as in the CoreDNS deployment repository.
**Release note**:

```release-note
Kubeadm: Make CoreDNS run in read-only mode and drop all unneeded privileges 
```