When a Dockerfile command requires using the TakeSnapshotFS function,
the resulting layer has a random ordering of files. This causes the
layer to have a non-deterministic hash defeating the reproducible flag.
Issue #710 appears to document this issue as well.
To fix, always sort the list of files to be added in scanFullFilesystem.
This avoids trying to sort the file list during execution, and takes
almost no time to complete.
619fc5e59b