Set ES_BEATS as environmennt variable for docker-compose (elastic#3299)

This allows to use ES_BEATS path to link containers in community beats

libbeat/scripts/Makefile

@@ -50,7 +50,7 @@ SYSTEM_TESTS?=false ## @testing if true, "make test" and "make testsuite" run un
 GOX_OS?=linux darwin windows solaris freebsd netbsd openbsd ## @Building List of all OS to be supported by "make crosscompile".
 TESTING_ENVIRONMENT?=snapshot ## @testing The name of the environment under test
 DOCKER_COMPOSE_PROJECT_NAME?=${BEATNAME}_${TESTING_ENVIRONMENT} ## @testing The name of the docker-compose project used by the integration and system tests
-DOCKER_COMPOSE?=TESTING_ENVIRONMENT=${TESTING_ENVIRONMENT} docker-compose -p ${DOCKER_COMPOSE_PROJECT_NAME} -f docker-compose.yml
+DOCKER_COMPOSE?=TESTING_ENVIRONMENT=${TESTING_ENVIRONMENT} ES_BEATS=${ES_BEATS} docker-compose -p ${DOCKER_COMPOSE_PROJECT_NAME} -f docker-compose.yml
 DOCKER_CACHE?=1 ## @miscellaneous If set to 0, all docker images are created without cache
 GOPACKAGES_COMMA_SEP=$(subst $(space),$(comma),$(strip ${GOPACKAGES}))
 PYTHON_ENV?=${BUILD_DIR}/python-env