CI/drone: use DRONE_TAG

.drone.yml

@@ -37,6 +37,8 @@ steps:
 
   - name: build
     image: golang:1.14
+    environment:
+      GIT_TAG: ${DRONE_TAG}
     commands:
       - make ci-setup
       - make build-cross

Makefile

@@ -16,7 +16,7 @@ export GO111MODULE=on
 ########## Tags ##########
 
 # get git tag
-GIT_TAG   := $(shell git describe --tags)
+GIT_TAG   ?= $(shell git describe --tags)
 ifeq ($(GIT_TAG),)
 GIT_TAG   := $(shell git describe --always)
 endif