Swap the default implementation now that batching is merged.
Keeping the `docker cp` based implementation around for the
moment, but it needs to be _explicitly_ disabled now by setting
`COMPOSE_EXPERIMENTAL_WATCH_TAR=0`.
After the next release, we should remove the `docker cp`
implementation entirely.
Signed-off-by: Milas Bowman <milas.bowman@docker.com>