Docker Support

Turns out there is no need to compress anything. Skopeo already compresses the layers.

Proposed patch would be a one-liner if I didn't hate the use of functools when shutils has a perfectly good copyfileobj.

diff --git a/plugins/pulp_docker/plugins/importers/upload.py b/plugins/pulp_docker/plugins/importers/upload.py
index 1ce47b0..780de62 100644
--- a/plugins/pulp_docker/plugins/importers/upload.py
+++ b/plugins/pulp_docker/plugins/importers/upload.py
@@ -22,6 +22,7 @@ import functools
 import gzip
 import json
 import os
+import shutil
 import stat
 import tarfile

@@ -340,11 +341,8 @@ class AddUnits(PluginStep):
             blob_src_path = os.path.join(self.get_working_dir(), item.digest.split(':')[1] + '.tar')
             blob_dest_path = os.path.join(self.get_working_dir(), item.digest)
             with open(blob_src_path) as blob_src:
-                with contextlib.closing(gzip.open(blob_dest_path, 'w')) as blob_dest:
-                    # these can be big files, so we chunk them
-                    reader = functools.partial(blob_src.read, 4096)
-                    for chunk in iter(reader, ''):
-                        blob_dest.write(chunk)
+                with contextlib.closing(open(blob_dest_path, 'w')) as blob_dest:
+                    shutil.copyfileobj(blob_src, blob_dest)
             # we don't need the tarfile anymore
             os.remove(blob_src_path)

Steps to reproduce and verify:
https://github.com/pulp/pulp_docker/pull/211#issuecomment-344270012

@preethi I don't think this issue should be automated at this time. Doing so requires adding a dependency on skopeo, and we have enough other things to do without adding another dependency.