Add GOLANG_VERSION arg support to proxy-docker make target + Dockerfile (#1430)

* Add GOLANG_VERSION arg to proxy-docker

* Set default GOLANG_VERSION to 1.13 in Makefile

* Add note in proxy Dockerfile re: GOLANG_VERSION

* Add GOLANG_VERSION to docker-compose dev target

docker-compose.yml

@@ -4,6 +4,8 @@ services:
     build:
       context: .
       dockerfile: cmd/proxy/Dockerfile
+      args:
+        GOLANG_VERSION: 1.13
     environment:
       - ATHENS_MONGO_STORAGE_URL=mongodb://mongo:27017
       - TIMEOUT=20 # in case the mongo dependency takes longer to start up