Accept vanilla Linux Foundation signoffs

This allows signing off commits with `git commit -s`

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

Cleaned up DCO regex
Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)

contrib/prepare-commit-msg.hook

@@ -1,10 +0,0 @@
-#!/bin/sh
-#       Auto sign all commits to allow them to be used by the Docker project.
-#       see https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
-#
-GH_USER=$(git config --get github.user)
-SOB=$(git var GIT_AUTHOR_IDENT | sed -n "s/^\(.*>\).*$/Docker-DCO-1.1-Signed-off-by: \1 \(github: $GH_USER\)/p")
-grep -qs "^$SOB" "$1" || { 
-	echo 
-	echo "$SOB" 
-} >> "$1"

hack/make/validate-dco

@@ -13,8 +13,8 @@ notDocs="$(validate_diff --numstat | awk '$3 !~ /^docs\// { print $3 }')"
 githubUsernameRegex='[a-zA-Z0-9][a-zA-Z0-9-]+'
 
 # https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
-dcoPrefix='Docker-DCO-1.1-Signed-off-by:'
-dcoRegex="^$dcoPrefix ([^<]+) <([^<>@]+@[^<>]+)> \\(github: ($githubUsernameRegex)\\)$"
+dcoPrefix='Signed-off-by:'
+dcoRegex="^(Docker-DCO-1.1-)?$dcoPrefix ([^<]+) <([^<>@]+@[^<>]+)>( \\(github: ($githubUsernameRegex)\\))?$"
 
 check_dco() {
 	grep -qE "$dcoRegex"