add genuser.sh, fix errors

macOS and Windows might not have htpasswd tools available, so adding this to the tpotinit image. users can run genuser.sh which simply contains a docker command to run tpotinit to create a user and add it to the T-Pot config (.env).
Fix an issue where WEB_USER was added with hyphens
Fix issues where shebang was incorrect
Update .env / env.example

docker/tpotinit/dist/entrypoint.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 COMPOSE="/tmp/tpot/docker-compose.yml"
 exec > >(tee /data/tpotinit.log) 2>&1