Encode the tar ball with gzip

Gzipping this tarball can greatly reduce the time it takes to send a docker file.
Signed-off-by: speedplane <mes65@cornell.edu>

docker/api/build.py

@@ -46,7 +46,8 @@ class BuildApiMixin(object):
             if os.path.exists(dockerignore):
                 with open(dockerignore, 'r') as f:
                     exclude = list(filter(bool, f.read().splitlines()))
-            context = utils.tar(path, exclude=exclude, dockerfile=dockerfile)
+            context = utils.tar(path, exclude=exclude, dockerfile=dockerfile, gzip=True)
+            encoding = 'gizp'
 
         if utils.compare_version('1.8', self._version) >= 0:
             stream = True