CORE_VERSION: Defer assignment until after submodules are checked out

Makefile

@@ -45,7 +45,7 @@ ACTIVATE_VENV = $(BIN)/activate
 PYTHON_VERSION := $(shell $(PYTHON) -c 'import sys; print("%u.%u" % (sys.version_info.major, sys.version_info.minor))')
 
 # core version to ensure docker images are based on the latest tagged release
-CORE_VERSION := $(shell git -C core describe --tags --abbrev=0)
+CORE_VERSION = $(shell git -C core describe --tags --abbrev=0)
 
 define SEMGIT
 $(if $(shell sem --version 2>/dev/null),sem -q --will-cite --fg --id ocrd_all_git,$(error cannot find package GNU parallel))