HTTP Transport compression for Layer downloads

[stevvooe]

We should add transport layer compression support for the registry server. This should be compliant with RFC7231, Section 5.3.4. This can be implemented as a wrapper around the main application that detects Accept-Encoding headers and acts on the contents.

Supersedes docker-archive/docker-registry#694.

[RichardScothern]

After some research on the subject I don't think this is worth doing. Mainly because tarsum supports - and enables - compression so layers pushed to a repository are already compressed with gzip.

There is also some ambiguity about Content-Range and [non]compressed data. This is probably not a big deal, but will require more than transparently compressing responses.

more discussion here

ping @stevvooe

[stevvooe]

@RichardScothern Agreed. This is unfortunate. We may want to state this somewhere.