Check the logged user instead of $USER (#3330)

* Check the logged user instead of $USER

Given that `sudo usermod --login "$DOCKER_USER" coder` and `sudo groupmod -n "$DOCKER_USER" coder` modify the container's disk it'll persist across restarts, but environment variables will be reset to whatever state they had at the end of `Dockerfile`. In this case, `$USER` is set to `coder`, so this branch will always be true.

By checking with the output of `whoami`, which gets it's information from `/etc/passwd`, we make sure to get the real logged user and not the one defined by $USER.

We also move `USER="$DOCKER_USER"` out of the branch, since we always want this to happen at entry-point. If we don't do this assignment, $USER will contain `coder` upon restart.

* Update entrypoint.sh

Check `$DOCKER_USER` was defined before copying it to `$USER`.

ci/release-image/entrypoint.sh

@@ -5,16 +5,17 @@ set -eu
 # Otherwise the current container UID may not exist in the passwd database.
 eval "$(fixuid -q)"
 
-if [ "${DOCKER_USER-}" ] && [ "$DOCKER_USER" != "$USER" ]; then
+if [ "${DOCKER_USER-}" ]; then
+  USER="$DOCKER_USER"
+  if [ "$DOCKER_USER" != "$(whoami)" ]; then
     echo "$DOCKER_USER ALL=(ALL) NOPASSWD:ALL" | sudo tee -a /etc/sudoers.d/nopasswd >/dev/null
     # Unfortunately we cannot change $HOME as we cannot move any bind mounts
     # nor can we bind mount $HOME into a new home as that requires a privileged container.
     sudo usermod --login "$DOCKER_USER" coder
     sudo groupmod -n "$DOCKER_USER" coder
 
-  USER="$DOCKER_USER"
-
     sudo sed -i "/coder/d" /etc/sudoers.d/nopasswd
+  fi
 fi
 
 dumb-init /usr/bin/code-server "$@"