Use 'cached' consistency to improve file system performance on OS X

[egamma]

Docker provides different 'consistency' options to improve the file system access performance on non-Linux systems: https://docs.docker.com/docker-for-mac/osxfs-caching/ and https://blog.docker.com/2017/05/user-guided-caching-in-docker-for-mac/

I did a mini benchmark npm install gulp:

Mount Type
type=bind, consistency=consistent	32s
type=bind, consistency=cached	21s
type=bind, consistency=delegated	20s
type=volume	15s
Local fs access	11s

The consistency guarantees of consistency=cached are:

The host’s view of the mount is authoritative. There may be delays before updates made on the host are visible within a container

This aligns well with containers, where most of the work happens inside a container with occasional host access with a tool running outside the container. Also there can be no data loss when the container is killed.

Given the performance improvement the propsoal is to use consistency=cached by default for bind mounts.

In addition, we should expose a setting where the user can tune the consistency for development containers.