Fix auto_update DOCKER_REGISTRY default #1655

Job	Run time
build-docker (internet.nl, docker/Dockerfile, app)	2m 32s
build-docker (unbound, docker/Dockerfile, unbound)	1m 29s
build-docker (linttest, docker/Dockerfile, linttest)	3m 23s
build-docker (test-runner, docker/test-runner.Dockerfile)	4m 6s
build-docker (webserver, docker/webserver.Dockerfile)	1m 36s
build-docker (grafana, docker/grafana.Dockerfile)	1m 47s
build-docker (util, docker/util.Dockerfile)	1m 22s
lint/check	2m 44s
integration-test	5m 40s
docs	1s
development-environment-test	3m 52s
test	3m 57s
batch-integration-test	5m 23s
	37m 52s

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