Fix a mysterious Makefile issue

The GIT_REVISION variable was being set to "" by default.

docker/Makefile

@@ -1,6 +1,6 @@
 # These default settings can be overriden by exporting env variables
 
-GIT_REVISION    ?= $(git rev-parse HEAD)
+GIT_REVISION    ?= $(shell git rev-parse HEAD)
 
 NETWORK         ?= testnet1
 NETWORK_TYPE    ?= libp2p
@@ -8,7 +8,7 @@ NETWORK_TYPE    ?= libp2p
 IMAGE_TAG       ?= $(NETWORK)
 IMAGE_NAME      ?= statusteam/nimbus_beacon_node:$(IMAGE_TAG)
 
-build: $(NIX_INSTALL)
+build:
 	docker build \
 		--build-arg="GIT_REVISION=$(GIT_REVISION)" \
 		--build-arg="NETWORK=$(NETWORK)" \