bug 381 fix

- fixup ping_pong Makefile
Signed-off-by: Mike Marchetti <mmarchetti@sandvine.com>
diff --git a/descriptor-packages/mk/pkg.mk b/descriptor-packages/mk/pkg.mk
index 0264602..6a2b553 100644
--- a/descriptor-packages/mk/pkg.mk
+++ b/descriptor-packages/mk/pkg.mk
@@ -24,7 +24,7 @@
 CHARM_SRC_DIR    := $(CHARM_DIR)/layers
 CHARM_DOCKER_TAG := charm-tools
 CHARM_BUILD_DIR  := $(CHARM_DIR)/builds
-DOCKER_BUILD     ?= $(shell which docker)
+DOCKER_BUILD     ?=
 
 Q=@