diff --git a/components/engine/Makefile b/components/engine/Makefile
index 403c6792ee..7767409a6f 100644
--- a/components/engine/Makefile
+++ b/components/engine/Makefile
@@ -13,6 +13,12 @@ DOCKERFILE := $(shell bash -c 'source hack/make/.detect-daemon-osarch && echo $$
 DOCKER_GITCOMMIT := $(shell git rev-parse --short HEAD || echo unsupported)
 export DOCKER_GITCOMMIT
 
+# allow overriding the repository and branch that validation scripts are running
+# against these are used in hack/validate/.validate to check what changed in the PR.
+export VALIDATE_REPO
+export VALIDATE_BRANCH
+export VALIDATE_ORIGIN_BRANCH
+
 # env vars passed through directly to Docker's build scripts
 # to allow things like `make KEEPBUNDLE=1 binary` easily
 # `project/PACKAGERS.md` have some limited documentation of some of these
@@ -49,6 +55,9 @@ DOCKER_ENVS := \
 	-e TESTDIRS \
 	-e TESTFLAGS \
 	-e TIMEOUT \
+	-e VALIDATE_REPO \
+	-e VALIDATE_BRANCH \
+	-e VALIDATE_ORIGIN_BRANCH \
 	-e HTTP_PROXY \
 	-e HTTPS_PROXY \
 	-e NO_PROXY \
diff --git a/components/engine/hack/validate/.validate b/components/engine/hack/validate/.validate
index 32cb6b6d64..6ed3a59a73 100644
--- a/components/engine/hack/validate/.validate
+++ b/components/engine/hack/validate/.validate
@@ -6,13 +6,16 @@ if [ -z "$VALIDATE_UPSTREAM" ]; then
 	# this is kind of an expensive check, so let's not do this twice if we
 	# are running more than one validate bundlescript
 
-	VALIDATE_REPO='https://github.com/docker/docker.git'
-	VALIDATE_BRANCH='master'
+	VALIDATE_REPO="${VALIDATE_REPO:-https://github.com/docker/docker.git}"
+	VALIDATE_BRANCH="${VALIDATE_BRANCH:-master}"
 
 	VALIDATE_HEAD="$(git rev-parse --verify HEAD)"
 
-	git fetch -q "$VALIDATE_REPO" "refs/heads/$VALIDATE_BRANCH"
-	VALIDATE_UPSTREAM="$(git rev-parse --verify FETCH_HEAD)"
+	if [ -z "$VALIDATE_ORIGIN_BRANCH" ]; then
+		git fetch -q "$VALIDATE_REPO" "refs/heads/$VALIDATE_BRANCH"
+		VALIDATE_ORIGIN_BRANCH=FETCH_HEAD
+	fi
+	VALIDATE_UPSTREAM="$(git rev-parse --verify $VALIDATE_ORIGIN_BRANCH)"
 
 	VALIDATE_COMMIT_LOG="$VALIDATE_UPSTREAM..$VALIDATE_HEAD"
 	VALIDATE_COMMIT_DIFF="$VALIDATE_UPSTREAM...$VALIDATE_HEAD"