New comment by Piraty on void-packages repository

https://github.com/void-linux/void-packages/pull/21254#issuecomment-623942555

Comment:
because this came up in IRC again, here is my (naive) explanation.

### Explanation
- #19374 was clearly a user configuration issue. The user should have cleaned up like the error message indicates.
- by adding `-s=overlay2` to the service run file this switched (probably quite a few) users that were using a different storage driver before (such as devicemapper, probably old docker installs that defaulted to sth else than overlay2) to overlay2 storage driver
- now by my revert (7e4ec0a6a9ecb96b0f8a8364e19f465f9dca29ee) those users end up with two configured storage drivers (their old one and overlay2). thus docker daemon fail demanding to explicitly set the one to use clean up `/var/lib/docker` (just like the original author of #19374)

### how to fix
if you are affected,
- either clean up `/var/lib/docker` (remove all sotage driver dirs what are oboslete). might require migration of existing containers
- or put `OPTS="-s=STORAGEDRIVER"` (for example devimapper if that was used before) in `/etc/sv/docker/conf`