Makefile: Honour BIND_DIR from the environment

Setting BIND_DIR in the environment, as in `env BIND_DIR= make
test-integration`, had no effect: the Makefile assigned BIND_DIR with
`:=`, which overrides environment variables, so the source tree stayed
bind-mounted.

This was inconsistent with other variables and was easy to overlook.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

Makefile

@@ -85,7 +85,7 @@ DOCKER_ENVS := \
 # to allow `make BIND_DIR=. shell` or `make BIND_DIR= test`
 # (default to no bind mount if DOCKER_HOST is set)
 # note: BINDDIR is supported for backwards-compatibility here
-BIND_DIR := $(if $(BINDDIR),$(BINDDIR),$(if $(DOCKER_HOST),,.))
+BIND_DIR ?= $(if $(BINDDIR),$(BINDDIR),$(if $(DOCKER_HOST),,.))
 
 # DOCKER_MOUNT can be overridden, but use at your own risk!
 ifndef DOCKER_MOUNT