[mod] docker-entrypoint.sh - add script to make test.sh (shellcheck)

Add script docker-entrypoint.sh to shellcheck and try to _simplify_ and
_normalize_ some parts:

- fix issues reported by shellcheck
- don't mix tab and space indent
- command 'help' replaced by '-h': ./dockerfiles/docker-entrypoint.sh -h
- replace printf in help() by 'cat <<EOF'

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

Makefile

@@ -54,6 +54,8 @@ PHONY += test ci.test test.shell
 ci.test: test.pep8 test.pylint test.unit test.robot
 test:    test.pep8 test.pylint test.unit test.robot test.shell
 test.shell:
+	$(Q)shellcheck -x -s dash \
+		dockerfiles/docker-entrypoint.sh
 	$(Q)shellcheck -x -s bash \
 		utils/brand.env \
 		./manage \