Add README.md file for the docker directory

See issue #5873

Docker-DCO-1.1-Signed-off-by: Timothy <timothyhobbs@seznam.cz> (github: https://github.com/timthelion)
Add Dinesh Subhraveti to MAINTAINERS

Docker-DCO-1.1-Signed-off-by: Dinesh Subhraveti <dineshs@altiscale.com> (github: dineshs-altiscale)
Change 'Uploading context' wording

Docker-DCO-1.1-Signed-off-by: Ryan Stelly <ryan.stelly@live.com> (github: FLGMwt)
pkg/testutils: utility functions to facilitate writing Go tests

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Add Cristian as maintainer to pkg/testutils

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Fix format in maintainers files

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
filters, for images: start with untagged/tagged boolean

This is a new feature and flag. (replaces the suggestion of a flag for
--untagged images).
The concept is to have a syntax to filter. This begins with this
filtering for the 'images' subcommand, and at that only filtering for
whether images are untagged.
  example like: docker rmi $(docker images -q --filter 'untagged=true')

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
filters: remove out filter proc prototype

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
filters, for images: docs

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
filters: cleanup & fmt

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
images filter: remove the redundant inverted filter

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
images: fix markdown documentation

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
filter flag: split out for separate --filter flags

adding tests and allowing for easy passing of filters.Args from client
to server.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
server: very debug.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
filters: change untagged to dangling

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
filters: use json marshal, instead of beam/data

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
filters: don't encode an empty set. update comments

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Move duplicated FS "magic" values to the graphdriver package so they can be shared instead of duplicated

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
grammar; nothing big

Docker-DCO-1.1-Signed-off-by: Zac Dover <zdover@redhat.com> (github: zdover23)

Added another "The"

Docker-DCO-1.1-Signed-off-by: Zac Dover <zdover@redhat.com> (github: zdover23)
Merge pull request #6172 from tianon/remove-duplicated-magic-values

Move duplicated FS "magic" values to the graphdriver package so they can be shared