Consider sync.Pool for temporary objects

[jonjohnsonjr]

We allocate a lot of temporary things during a build, e.g. []byte buffers, gzip.Reader, gzip.Writer, bufio.Writer, bufio.Reader.

This is not a huge deal when building a single image, but the GC pressure is more noticeable when building a lot of images (as we do in the terraform provider).

We can alleviate a lot of this pressure by maintaining a few sync.Pools and reusing these objects wherever we can.