Make sure that build_script_files don't bust cache

This slowed down everything!

repo2docker/detectors.py

@@ -362,8 +362,21 @@ class BuildPack(LoggingConfigurable):
             io.BytesIO(dockerfile)
         )
 
+        def _filter_tar(tar):
+            # We need to unset these for build_script_files we copy into tar
+            # Otherwise they seem to vary each time, preventing effective use
+            # of the cache!
+            # https://github.com/docker/docker-py/pull/1582 is related
+            tar.uname = ''
+            tar.gname = ''
+            tar.uid = 1000
+            tar.gid = 1000
+            return tar
+
         for src in sorted(self.build_script_files):
-            tar.add(src)
+            src_parts = src.split('/')
+            src_path = os.path.join(os.path.dirname(__file__), 'files', *src_parts)
+            tar.add(src_path, src, filter=_filter_tar)
 
         tar.add('.', 'src/')
 
@@ -512,8 +525,8 @@ class CondaBuildPack(BuildPack):
     path = ['${CONDA_DIR}/bin']
 
     build_script_files = {
-        os.path.join(os.path.dirname(__file__), 'files', 'conda', 'install-miniconda.bash'): '/tmp/install-miniconda.bash',
-        os.path.join(os.path.dirname(__file__), 'files', 'conda', 'environment.yml'): '/tmp/environment.yml',
+        'conda/install-miniconda.bash': '/tmp/install-miniconda.bash',
+        'conda/environment.yml': '/tmp/environment.yml'
     }
 
     build_scripts = [