allow getting ssh user and password from environment variables

[Williangalvani]

I used this to install BlueOS on a Radxa Zero 3E running ubuntu.
The issue is there's no pi user there, so it requires these changes in order to be able to install the ssh key.
Additionaly, this is the updated startup.json:

{
    "core": {
        "binds": {
            "/dev/": {
                "bind": "/dev/",
                "mode": "rw"
            },
            "/etc/blueos": {
                "bind": "/etc/blueos",
                "mode": "rw"
            },
            "/etc/dhcpcd.conf": {
                "bind": "/etc/dhcpcd.conf",
                "mode": "rw"
            },
            "/etc/machine-id": {
                "bind": "/etc/machine-id",
                "mode": "ro"
            },
            "/etc/resolv.conf.host": {
                "bind": "/etc/resolv.conf.host",
                "mode": "ro"
            },
            "/home/pi/.ssh": {
                "bind": "/home/pi/.ssh",
                "mode": "rw"
            },
            "/home/rock/.ssh": {
                "bind": "/home/rock/.ssh",
                "mode": "rw"
            },
            "/run/udev": {
                "bind": "/run/udev",
                "mode": "ro"
            },
            "/sys/": {
                "bind": "/sys/",
                "mode": "rw"
            },
            "/tmp/wpa_playground": {
                "bind": "/tmp/wpa_playground",
                "mode": "rw"
            },
            "/usr/blueos/bin": {
                "bind": "/usr/blueos/bin",
                "mode": "rw"
            },
            "/usr/blueos/extensions": {
                "bind": "/usr/blueos/extensions",
                "mode": "rw"
            },
            "/usr/blueos/userdata": {
                "bind": "/usr/blueos/userdata",
                "mode": "rw"
            },
            "/var/logs/blueos": {
                "bind": "/var/logs/blueos",
                "mode": "rw"
            },
            "/var/run/dbus": {
                "bind": "/var/run/dbus",
                "mode": "rw"
            },
            "/var/run/docker.sock": {
                "bind": "/var/run/docker.sock",
                "mode": "rw"
            },
            "/var/run/wpa_supplicant": {
                "bind": "/var/run/wpa_supplicant",
                "mode": "rw"
            }
        },
        "enabled": true,
        "environment": [
            "SSH_USER=rock",
            "SSH_PASSWORD=rock",
            "USER_UID=1001",
            "USER_GID=1001",
            "BLUEOS_DISABLE_SERVICES=wifi,autopilot"
        ],
        "image": "williangalvani/blueos-core",
        "network": "host",
        "privileged": true,
        "tag": "user_pass",
        "webui": false
    }
}

For documentation sake:

• I'm using radxa-zero3_ubuntu_jammy_cli_b6.img.xz
• I just logged in and run the install script, then updated the image until it could setup ssh properly
• debian and cli images dont seem to be working at all on the radxa zero 3e

potentially fix #2984

[joaoantoniocardoso]

Just fix the fallbacks