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`.