When building from same stream all nodes need to read
data from the same stream. In order to achive that there
is a new SyncMultiReader wrapper that sends the stream
concurrently to all readers. Readers must read at similar
speed or pauses will happen while they wait for each other.
Dockerfiles were already written to disk before sent. Now
the file written by first node is reused for others.
Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
