Introduce an arch field that can be used to filter repositories based on
the local architecture.
If arch is provided and matching with the current GOARCH then the
repository is enabled
docker.Untar
(https://github.com/mudler/luet/blob/master/vendor/github.com/docker/docker/pkg/archive/archive.go#L942) requires absolute paths.
We didn't do any input validation before, assuming the path passed by
were absolute since they were coming from YAML configuration files, now
that this is not the truth anymore we need to sanitize the input.
With this change we check if the given path is absolute or relative, if
it's relative we calculate the absolute path and use it in place.
