Avoid erasing the developers' Docker config on local builds

ci/build.sh

@@ -20,10 +20,12 @@ if [ -z "$VERSION" -o -z "$CI_PROJECT_DIR" -o -z "$CI_REGISTRY_IMAGE" ]; then
 	exit 1
 fi
 
-# Authenticate on docker.io using common credentials to avoid quota issues
-mkdir -p ~/.docker || true
-echo "$DOCKER_AUTH_CONFIG" > ~/.docker/config.json
-docker login index.docker.io
+if [ -n "$DOCKER_AUTH_CONFIG" ]; then
+	# Authenticate on docker.io using common credentials to avoid quota issues
+	mkdir -p ~/.docker || true
+	echo "$DOCKER_AUTH_CONFIG" > ~/.docker/config.json
+	docker login index.docker.io
+fi
 
 if [ -n "$CI_REGISTRY" -a -n "$CI_REGISTRY_USER" -a -n "$CI_REGISTRY_PASSWORD" ]; then
 	echo Logging in to container registry…