Change cert path to /etc/docker

Closes #9468 

ping @dmcgowan 

thanks @lsm5, hopefully derek will let us know if this causes any other damage ;)


this path obviously won't work on Windows. even though Uid=="0" check will only pass on Unix it's not explicit that how different environments are handled here. Perhaps we can have a platform-specific `getCertDir()` implementation (just like `getHomeDir()`) and in there we can explicitly handle root/Administrator checks. Right now Uid=="0" check is not a valid check for Windows but it will be executed on Windows anyway.


I'm not sure what this change is going to achieve with /etc/hosts. Could you please elaborate, perhaps then we can suggest some Windows-equivalent path providing similar guarantees?  @icecrime @johngossman