Added proposed Docker Community Guidelines

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
Rename DOCKER_CONFIG envvar to DOCKER_CERT_PATH

DOCKER_CONFIG was introduced in #6984.

We may use "config" for other purposes (e.g. #7232). Until we
have made a design decision around how configuration files will
work, DOCKER_CERT_PATH is a much safer name to rely on for future
compatibility.

Docker-DCO-1.1-Signed-off-by: Ben Firshman <ben@firshman.co.uk> (github: bfirsh)
Merge pull request #7412 from bfirsh/rename-docker-config

Rename DOCKER_CONFIG envvar to DOCKER_CERT_PATH
Merge pull request #7186 from jamtur01/community

Added proposed Docker Community Guidelines
Automatically relabel volume mounts if user passes :Z with --volume

When you volume mount into a labeled container, we want to allow the
administrator/user the ability to tell docker to fix the labels on the mount.
Using the same basic syntax as the :ro and :rw options on the --volume command

:Z will set the labels so that only the processes within the container can
read/write content.  Since the labels change on creation, this can not be done
by the administrator prior to running the container

:z will change the labels so all containers could read/write content.

drwxr-xr-x. root root system_u:object_r:svirt_sandbox_file_t:s0 /var/run/dan

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)