Fix sed command to remove last line in sudoers file

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

Author: stuartpb

ci/release-image/entrypoint.sh

@@ -14,7 +14,7 @@ if [ "${DOCKER_USER-}" ]; then
     sudo usermod --login "$DOCKER_USER" coder
     sudo groupmod -n "$DOCKER_USER" coder
 
-    sudo sed -i "/coder/d" /etc/sudoers.d/nopasswd
+    sudo sed -i '$d' /etc/sudoers.d/nopasswd
   fi
 fi