Add comments explaining what entrypoint is supposed to do

[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

[stuartpb]

We create the coder user as part of the image so it will always be the right user to remove

Okay, so now imagine the crazy hypothetical scenario where literally any part of that very specific scaffolding for this specific fork of the codebase provided by the outside environment changes, for any reason, like if, I dunno, the name of the company changed and you missed one or two extant references to it.

[stuartpb]

Or rather, sorry, I guess I just don't understand how any of these variables are expected to work because there are zero usable comments here. The idea is that the line we're deleting isn't supposed to be deleting the user named by DOCKER_USER? How am I supposed to understand what this code's supposed to be doing?

[stuartpb]

Here, I've updated the PR, here's a version where you can tell what cases would and wouldn't have bugs because the code actually describes what it's trying to do.