I'm about to start interacting with docker for Debian packaging too,
so it's time to centralize this so that any bugfixes I figure out
apply to both codepaths.
Simplifies the rpm build process.
We will use platform specific rpmbuild directories and will not clean them and
will drop the explicit copy to build directory.
Docker can be run by ordinary users if they are in the docker group. The build
process would however be run as a root user, only protected by the sandboxing.
That caused problems with the shared directory where rpmbuild would be picky
about building from sources owned by less privileged users and producing files
owned by root.
Instead, add a build user with the right uid/gid to the image and run the
docker process as that user.
