Restore DEBUG as DOCKER_DEBUG (removed in #10665), add as make variable

Signed-off-by: Aidan Hobson Sayers <aidanhs@cantab.net>

Makefile

@@ -6,6 +6,7 @@
 DOCKER_ENVS := \
 	-e BUILDFLAGS \
 	-e DOCKER_CLIENTONLY \
+	-e DOCKER_DEBUG \
 	-e DOCKER_EXECDRIVER \
 	-e DOCKER_GRAPHDRIVER \
 	-e DOCKER_STORAGE_OPTS \

hack/make.sh

@@ -122,7 +122,9 @@ fi
 
 IAMSTATIC='true'
 source "$SCRIPTDIR/make/.go-autogen"
-LDFLAGS='-w'
+if [ -z "$DOCKER_DEBUG" ]; then
+	LDFLAGS='-w'
+fi
 
 LDFLAGS_STATIC='-linkmode external'
 # Cgo -H windows is incompatible with -linkmode external.