make docker: pass in VCS_REF and BUILD_DATE, too

Makefile

@@ -1,6 +1,7 @@
 PROJECT_NAME := ocrd_fileformat
 TOOLS = ocrd-fileformat-transform
-DOCKER_TAG = ocrd/fileformat
+DOCKER_BASE_IMAGE ?= docker.io/ocrd/core:v2.69.0
+DOCKER_TAG ?= ocrd/fileformat
 
 PIP ?= pip3
 
@@ -67,7 +68,11 @@ uninstall:
 
 # Build Docker image
 docker:
-	docker build -t '$(DOCKER_TAG)' .
+	docker build \
+	--build-arg DOCKER_BASE_IMAGE=$(DOCKER_BASE_IMAGE) \
+	--build-arg VCS_REF=$$(git rev-parse --short HEAD) \
+	--build-arg BUILD_DATE=$$(date -u +"%Y-%m-%dT%H:%M:%SZ") \
+	-t $(DOCKER_TAG) .
 
 #
 # Assets