"Failed to save config" error
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**Describe Your Problem:**
<!-- A clear and concise description of what problem you are trying to solve. -->
When trying to save settings on a plugin through the Homebridge Config UI I get an error stating "Failed to save config". A things to notice:

- I configured the UID as 555 which is my uid on MacOS.
- A shared a few files/directories with the host (see docker-compose.yml)

**Logs:**
<!-- Support requests that do not contain logs may be closed without warning. -->

```
❯ docker-compose up
Starting homebridge ... done
Attaching to homebridge
homebridge    | [s6-init] making user provided files available at /var/run/s6/etc...exited 0.
homebridge    | [s6-init] ensuring user provided files have correct perms...exited 0.
homebridge    | [fix-attrs.d] applying ownership & permissions fixes...
homebridge    | [fix-attrs.d] done.
homebridge    | [cont-init.d] executing container initialization scripts...
homebridge    | [cont-init.d] 10-adduser: executing... 
homebridge    | usermod: no changes
homebridge    | 
homebridge    | -------------------------------------
homebridge    | GID/UID
homebridge    | -------------------------------------
homebridge    | User uid:    555
homebridge    | User gid:    1000
homebridge    | -------------------------------------
homebridge    | 
homebridge    | [cont-init.d] 10-adduser: exited 0.
homebridge    | [cont-init.d] 20-set-timezone: executing... 
homebridge    | [cont-init.d] 20-set-timezone: exited 0.
homebridge    | [cont-init.d] 30-packages: executing... 
homebridge    | [cont-init.d] 30-packages: exited 0.
homebridge    | [cont-init.d] 40-dbus-avahi: executing... 
homebridge    | [cont-init.d] 40-dbus-avahi: exited 1.
homebridge    | [cont-init.d] 45-user-data: executing... 
homebridge    | [cont-init.d] 45-user-data: exited 0.
homebridge    | [cont-init.d] 50-plugins: executing... 
homebridge    | 
homebridge    | 
homebridge    |     Thank you for using the oznu/homebridge docker image!
homebridge    | 
homebridge    |   If you find this project useful please STAR it on GitHub:
homebridge    | 
homebridge    |          https://github.com/oznu/docker-homebridge
homebridge    | 
homebridge    | 
homebridge    | Homebridge: Installing plugins...
homebridge    | up to date in 0.33s
homebridge    | up to date in 0.446s
homebridge    | [cont-init.d] 50-plugins: exited 0.
homebridge    | [cont-init.d] 55-config-ui: executing... 
homebridge    | Enabling homebridge-config-ui-x...
homebridge    | [cont-init.d] 55-config-ui: exited 0.
homebridge    | [cont-init.d] done.
homebridge    | [services.d] starting services
homebridge    | Starting cron
homebridge    | [10/12/2020, 12:37:59 PM] [Config] Password: ***************
homebridge    | 
homebridge    | 
homebridge    |     Thank you for using the oznu/homebridge docker image!
homebridge    | 
homebridge    |   If you find this project useful please STAR it on GitHub:
homebridge    | 
homebridge    |          https://github.com/oznu/docker-homebridge
homebridge    | 
homebridge    | 
homebridge    | [services.d] done.
homebridge    | [10/12/2020, 12:49:04 PM] [HB Supervisor] Homebridge Storage Path: /homebridge
homebridge    | [10/12/2020, 12:49:04 PM] [HB Supervisor] Homebridge Config Path: /homebridge/config.json
homebridge    | [10/12/2020, 12:49:04 PM] [HB Supervisor] Logging to /homebridge/homebridge.log
homebridge    | [10/12/2020, 12:49:04 PM] [HB Supervisor] OS: Linux 4.19.76-linuxkit x64
homebridge    | [10/12/2020, 12:49:04 PM] [HB Supervisor] Node.js v12.18.4 /usr/local/bin/node
homebridge    | [10/12/2020, 12:49:04 PM] [HB Supervisor] Homebridge Path: /usr/local/lib/node_modules/homebridge/bin/homebridge
homebridge    | [10/12/2020, 12:49:04 PM] [HB Supervisor] UI Path: /usr/local/lib/node_modules/homebridge-config-ui-x/dist/bin/standalone.js
homebridge    | [10/12/2020, 12:49:04 PM] [HB Supervisor] Starting Homebridge with extra flags: -I -P /homebridge/node_modules
homebridge    | [10/12/2020, 12:49:05 PM] [HB Supervisor] Started Homebridge v1.2.3 with PID: 356
homebridge    | [10/12/2020, 12:49:05 PM] Loaded config.json with 0 accessories and 1 platforms.
homebridge    | [10/12/2020, 12:49:05 PM] ---
homebridge    | [10/12/2020, 12:49:06 PM] Loaded plugin: homebridge-dummy@0.4.1
homebridge    | [10/12/2020, 12:49:06 PM] Registering accessory 'homebridge-dummy.DummySwitch'
homebridge    | [10/12/2020, 12:49:06 PM] ---
homebridge    | [10/12/2020, 12:49:06 PM] Loaded plugin: homebridge-config-ui-x@4.27.2
homebridge    | [10/12/2020, 12:49:06 PM] Registering platform 'homebridge-config-ui-x.config'
homebridge    | [10/12/2020, 12:49:06 PM] ---
homebridge    | [10/12/2020, 12:49:06 PM] Loading 1 platforms...
homebridge    | [10/12/2020, 12:49:06 PM] [Config] Initializing config platform...
homebridge    | [10/12/2020, 12:49:06 PM] [Config] Running in Service Mode
homebridge    | Setup Payload:
homebridge    | X-HM://***************
homebridge    | Enter this code with your HomeKit app on your iOS device to pair with Homebridge:
homebridge    |                        
homebridge    |     ┌────────────┐     
homebridge    |     │ ***-**-*** │     
homebridge    |     └────────────┘     
homebridge    |                        
homebridge    | [10/12/2020, 12:49:06 PM] Homebridge v1.2.3 is running on port 52462.
homebridge    | [10/12/2020, 12:49:07 PM] [Config] Homebridge Config UI X v4.27.2 is listening on 0.0.0.0 port 8080
```

**Docker Start Command:**
<!-- Show how you are starting the docker container -->
<!-- docker-compose? provide the full docker-compose.yml file -->

```
version: '2'
services:
  homebridge:
    image: oznu/homebridge:latest
    container_name: homebridge
    restart: always
    environment:
      - TZ=Europe/Amsterdam
      - PGID=1000
      - PUID=555
      - HOMEBRIDGE_CONFIG_UI=1
      - HOMEBRIDGE_CONFIG_UI_PORT=8080
    volumes:
      - ./conf/config.json:/homebridge/config.json:rw
      - ./package.json:/homebridge/package.json:rw
      - ./node_modules/:/homebridge/node_modules/:rw
    ports:
      - 8080:8080

```

**Homebridge Config:**

```json
{
    "bridge": {
        "name": "Homebridge *******",
        "username": "**:**:**:**:**:**",
        "port": *****,
        "pin": "***-**-***"
    },
    "accessories": [],
    "platforms": [
        {
            "name": "Config",
            "port": 8080,
            "platform": "config"
        }
    ]
}
```

**Screenshots:**
<!-- If applicable, add screenshots to help explain your problem. -->

![image](https://user-images.githubusercontent.com/1086908/95739398-16f3dc80-0c8b-11eb-9029-c0cefbe8a8b1.png)


**Environment:**

* **Host Operating System**: macOS
* **Docker Version**: Docker version 19.03.13, build 4484c46d9d
* **Image Tag / Type**: oznu/homebridge:latest

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**Additional thoughts**

I see that `hb-service` is running as root instead of `abc` user. Could this cause the permission error?

```
❯ docker-compose exec homebridge ash
/homebridge # ps
PID   USER     TIME  COMMAND
    1 root      0:00 s6-svscan -t0 /var/run/s6/services
   30 root      0:00 s6-supervise s6-fdholderd
   31 root      0:00 s6-supervise docker-cmd
  314 root      0:00 s6-supervise cron
  315 root      0:00 s6-supervise hb-service-logs
  316 root      0:00 s6-supervise homebridge-config-ui-x
  318 root      0:00 /usr/sbin/crond -f -S -l 5 -c /etc/crontabs
  321 abc       0:00 tail -f --follow=name /homebridge/homebridge.log
  322 root      0:00 sh /usr/bin/with-localenv sh ./run
  330 root      0:00 sh ./run
===> 344 root      0:03 {node} hb-service <===
  356 abc       0:00 {node} homebridge
  393 root      0:00 ash
  399 root      0:00 ps
```

Shouldn't this line:
https://github.com/oznu/docker-homebridge/blob/master/root/etc/services.d/homebridge-config-ui-x/run#L16

Be "s6-setuidgid **abc** hb-service run \ ... " ?