Fix sed command to remove last line in sudoers file

[stuartpb]

This way the DOCKER_USER variable won't accidentally retain sudoer permissions for any user not named coder (almost certainly a security hole).

[code-asher]

How would this be reproduced? We create the coder user as part of the image so it will always be the right user to remove:

code-server/ci/release-image/Dockerfile

Lines 37 to 38 in 53d981a

	&& adduser --gecos '' --disabled-password coder \
	&& echo "coder ALL=(ALL) NOPASSWD:ALL" >> /etc/sudoers.d/nopasswd

If the user is not coder somehow, then these previous lines trying to rename it will fail anyway.

code-server/ci/release-image/entrypoint.sh

Lines 14 to 15 in 53d981a

	sudo usermod --login "$DOCKER_USER" coder
	sudo groupmod -n "$DOCKER_USER" coder