add some info wrt backup and restore of volumes

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Check if the command exists before dereference

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
Merge pull request #4394 from creack/fix_tty_ghost

Check if the command exists before dereference
Make the chan for utils.Go buffered in order to avoid goroutine leak

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
Merge pull request #4395 from creack/remove_goroutine_leak_test

Make the chan for utils.Go buffered in order to avoid goroutine leak
Add fish shell completions from https://github.com/barnybug/docker-fish-completion.

Docker-DCO-1.1-Signed-off-by: Barnaby Gray <barnaby@pickle.me.uk> (github: barnybug)
adding note that host-volumes must be defined using absolute paths

Docker-DCO-1.1-Signed-off-by: Will Weaver <monkey@buildingbananas.com> (github: funkymonkeymonk)
DOC: Fix switch name for tagging images in build command

Docker-DCO-1.1-Signed-off-by: German Del Zotto <germ@ndz.com.ar> (github: GermanDZ)
mkdir /dev before doing mknod

Docker-DCO-1.1-Signed-off-by: Lewis Peckover <lew@lew.io> (github: lewispeckover)
Merge pull request #4411 from lewispeckover/fix_mkimage-yum_mknod

Add "mkdir /dev" before doing mknod in mkimage-yum
Merge pull request #4399 from barnybug/fish-completions

Add fish shell completions from github.com/barnybug/docker-fish-completion
Merge pull request #4176 from SvenDowideit/how-to-backup-volumes

add some info wrt backup and restore of volumes
Merge pull request #4410 from 1uptalent/fix_typo_in_build_command_doc

DOC: Fix switch name for tagging images in build command
Merge pull request #4406 from funkymonkeymonk/voldocs

adding note that host-volumes must be defined using absolute paths