docker: use latest core base stage

Makefile

@@ -3,8 +3,9 @@ PIP = pip3
 PYTHONIOENCODING=utf8
 PYTEST_ARGS = -vv
 
-DOCKER_BASE_IMAGE = docker.io/ocrd/core:v3.3.0
+DOCKER_BASE_IMAGE ?= docker.io/ocrd/core:latest
-DOCKER_TAG = ocrd/dinglehopper
+DOCKER_TAG ?= ocrd/dinglehopper
+DOCKER ?= docker
 
 help:
 	@echo
@@ -24,7 +25,7 @@ test:
 	pytest $(PYTEST_ARGS)
 
 docker:
-	docker build \
+	$(DOCKER) build \
 	--build-arg DOCKER_BASE_IMAGE=$(DOCKER_BASE_IMAGE) \
 	--build-arg VCS_REF=$$(git rev-parse --short HEAD) \
 	--build-arg BUILD_DATE=$$(date -u +"%Y-%m-%dT%H:%M:%SZ") \