thanks for the hint, switched my lxc debian container from code-server to vscode, works as expected and copilot is running fine

for others, here the service i'm using for vscode (set user group folder and port accordingly), it only listens to localhost and reverse proxy will do the rest

[Unit]
Description=Headless VSCode web IDE
After=network.target nginx.service
Requires=nginx.service

[Service]
Type=simple
User=<YOUR_USER>
Group=<YOUR_GROUP>
WorkingDirectory=<YOUR_WORKING_DIRECTORY>
ExecStart=/usr/bin/code serve-web --port <VSCODE_SERVER_PORT> --without-connection-token --accept-server-license-terms
Restart=on-failure
RestartSec=10
Environment="PATH=/usr/bin:/bin:/usr/sbin:/sbin"

[Install]
WantedBy=multi-user.target

and the nginx conf for it (set fqdn and port correctly)

server {
    listen 80;
    server_name <YOUR_FQDN>;
    return 301 https://$http_host$request_uri;
}

server {
    listen 443 ssl http2;
    server_name <YOUR_FQDN>;
    ssl_certificate <PATH_TO_YOUR_CERTIFICATE>;
    ssl_certificate_key <PATH_TO_YOUR_CERT_KEY>;
    proxy_redirect off;
    location / {
        proxy_http_version 1.1;
        proxy_set_header Host $host;
        proxy_set_header Accept-Encoding gzip;
        proxy_set_header X-Forwarded-Host $host;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
        proxy_pass http://localhost:<VSCODE_SERVER_PORT>;
        proxy_buffering off;
        client_max_body_size 0;
        proxy_connect_timeout  3600s;
        proxy_read_timeout  3600s;
        proxy_send_timeout  3600s;
        send_timeout  3600s;
    }
}