Synthesize executable bit on Windows

The build context is tarred up on the client and then sent to the Docker
daemon.  However Windows permissions don't match the Unix ones.

Therefore we have to mark all files as executable when creating a build
context on Windows, like `docker build` already does:
https://github.com/docker/docker/issues/11047.
Signed-off-by: Sebastian Schwarz <seschwar@gmail.com>

docker/utils/utils.py

@@ -4,6 +4,7 @@ import os
 import os.path
 import json
 import shlex
+import sys
 import tarfile
 import tempfile
 import warnings
@@ -92,7 +93,10 @@ def tar(path, exclude=None, dockerfile=None, fileobj=None, gzip=False):
     exclude = exclude or []
 
     for path in sorted(exclude_paths(root, exclude, dockerfile=dockerfile)):
-        t.add(os.path.join(root, path), arcname=path, recursive=False)
+        i = t.gettarinfo(os.path.join(root, path), arcname=path)
+        if sys.platform == 'win32':
+            i.mode = i.mode & 0o755 | 0o111
+        t.addfile(i)
 
     t.close()
     fileobj.seek(0)