Update entrypoint.sh to allow for user management via environment variables

[Derekt2]

Allows user management purely from environment variables. This allows anyone deploying the server the ability to add or remove users easily. If you unset the environment variable and redeploy, it removes all users and adds admin if it doesn't already exists so there will always be at least one user.

[Derekt2]

can be tested easily with this docker-compose file, then verifying by connecting to localhost.

version: "3"

services:
  server:
    build:
      context: .
      dockerfile: Dockerfile
    command: server
    environment:
      - GHIDRA_IP=127.0.0.1
      - GHIDRA_USERS=admin user1 user2
    volumes:
      - ./repos:/repos:rw
    ports:
      - 13100:13100
      - 13101:13101
      - 13102:13102