Separate init script from the start script

[dezeroku]

In k8s environments it's quite common to bootstrap the deployments following these steps:

1. Initialize the environment using upstream container
2. Apply local customizations with a script
3. Run the upstream container "as usual"

Currently there's no easy way to perform the initial configuration without starting the main processes, this PR separates "init" and "run" parts.

The init.sh file is basically copied as-is from the start.sh with two changes:

1. removed the unnecessary backslash with a trailing space after then in line 30, as it was causing /start.sh: line 30: : not found warning/error
2. wrapped echoed variable in double quotes in line 70

[antonym]

What's the benefit in splitting up the file? Is that so the entry point can be overridden? I'm open to the change, would just like to understand it better.

[dezeroku]

Yes, overriding the entrypoint is basically the reason. With this split it's easy to automate:

1. obtaining the config
2. applying local changes
3. starting the service

For example, let's say that you want to use a local mirror by setting the live_endpoint entry. Currently to achieve it you have to:

1. start the container (to obtain the configuration files)
2. stop it manually
3. modify the config file
4. start it again (to serve the modified content).

With this PR merged you could automate this with a "middle-man" container, like so:.

1. start the container with entrypoint /init.sh (to obtain the configuration files)
2. modify the config file (e.g. with sed)
3. start the container with default entrypoint (to serve the modified content)

An example docker-compose file using this scheme would look like that:

---
version: "2.1"
services:
  netbootxyz-init:
    image: ghcr.io/netbootxyz/netbootxyz
    environment:
      - MENU_VERSION=2.0.47
    volumes:
      - ./config:/config
      - ./assets:/assets
    command: /init.sh

  netbootxyz-customisations:
    image: busybox
    volumes:
      - ./config:/config
    command: sed 's#set live_endpoint.*#set live_endpoint http://my-custom-endpoint#' -i /config/menus/remote/boot.cfg
    depends_on:
        netbootxyz-init:
            condition: service_completed_successfully

  netbootxyz:
    image: ghcr.io/netbootxyz/netbootxyz
    container_name: netbootxyz
    environment:
      - NGINX_PORT=80
    volumes:
      - ./config:/config
      - ./assets:/assets
    ports:
      - 3000:3000
      - 69:69/udp
      - 8080:80
    restart: unless-stopped
    depends_on:
        netbootxyz-customisations:
            condition: service_completed_successfully