Add utiity function to change permissions of user files created in a layer

[diegoferigo]

Commit 34f3ae9 introduced an initial support to handling better the permissions of files and folder that are used by the runtime user.

The resources chown is always demanded to downstream images. To streamline this operation we should introduce a new utility function that takes a folder and changes the group owner of all the files that are not already owned by the runtimeusers group.

Note that we have to filter the files with find -group root because chowning again a file in a layer would result in a dummy modification belonging to that layer. This operation would mark that file as modified (even if it was only touched by chown), making the image size increase without any reason.

[diegoferigo]

As a workaround, if a RUN statement creates resources that the runtime users has to access in r/w, using the following pattern prevents chowning:

USER root:runtimeusers
RUN # ...
USER root:root