/var/lib/derper/derper.key - to put pre-auth key in this file?

[europacafe]

I've headscale running as a container without problem. I also tried embedded derp with success.

I'm aiming to replace embedded derp with a separate derp container by using your derp image.

When spinning up the container, the log file said derper.key is not specified. So I create the derper.key on my host and put a newly created headscale pre-auth key in the file. The file contains just one line of pre-auth key:
58c263bc0e.............a073213bd2e

However, the log file now reports:

2025/07/24 06:45:17 no config path specified; using /var/lib/derper/derper.key
2025/07/24 06:45:17 derper: config: invalid character 'c' after top-level value

Below is my docker run:

docker run
  -d
  --name='derp'
  --net='bridge'
  --pids-limit 2048
  -e TZ="Asia/Bangkok"
  -e HOST_OS="Unraid"
  -e HOST_HOSTNAME="bthoven-unraid"
  -e HOST_CONTAINERNAME="derp"
  -e 'derp_DOMAIN'='derp.mydomain.com'
  -e 'TAILSCALE_DERP_VERIFY_CLIENTS'='true'
  -e 'derp_CERT_MODE'='manual'
  -l net.unraid.docker.managed=dockerman
  -p '7443:443/tcp'
  -p '7478:3478/udp'
  -v '/mnt/user/letsencrypt/mydomain.com.fullchain':'/app/certs/example.com.crt':'ro'
  -v '/mnt/user/letsencrypt/mydomain.com.key':'/app/certs/example.com.key':'ro'
  -v '/mnt/user/appdata/derp/derper.key':'/var/lib/derper/derper.key':'ro' 'ghcr.io/slchris/derp-server:v1'

I don't know what I have missed on this setup.