Using default value instead of simple variable for the git branch

Signed-off-by: Damien DUPORTAL <[email protected]>

Makefile

@@ -4,7 +4,7 @@ export DOCKER_IMAGE_NAME ?= dduportal/bats
 export DOCKER_IMAGE_TAG ?= $(shell git rev-parse --short HEAD)
 export BATS_VERSION ?= 0.4.0
 
-CURRENT_GIT_BRANCH=$(shell git rev-parse --abbrev-ref HEAD)
+CURRENT_GIT_BRANCH ?= $(shell git rev-parse --abbrev-ref HEAD)
 
 all: build test