RUN set -eux ; \
    cd /app/log/ ; \
    touch app.log ;\
    ln -sfT /dev/stdout app.log ; \
   chown -R --no-dereference www-data:www-data . ;

root@63f6b1c48b39:/app# ls -la /var/log/apache2/
total 8
drwxrwxrwx 2 www-data www-data 4096 May  8 02:37 .
drwxr-xr-x 1 root     root     4096 May  8 02:37 ..
lrwxrwxrwx 1 www-data www-data   11 May  8 02:37 access.log -> /dev/stdout
lrwxrwxrwx 1 www-data www-data   11 May  8 02:37 error.log -> /dev/stderr
lrwxrwxrwx 1 www-data www-data   11 May  8 02:37 other_vhosts_access.log -> /dev/stdout
root@63f6b1c48b39:/app# ls -la log/
total 8
drwxr-xr-x 1 www-data www-data 4096 May 28 13:55 .
drwxr-xr-x 1 root     root     4096 May 28 13:55 ..
lrwxrwxrwx 1 www-data www-data   11 May 28 13:55 app.log -> /dev/stdout

 if ($f = fopen($logfile, 'a')) {
    fwrite($f, strftime("%Y/%m/%d - %H:%M:%S: ") . $data);
    fclose($f);
    return true;
} else {
   return false;
}

Warning:  fopen(/app/log/app.log): failed to open stream: Permission denied in ..

version: '3'
services:
  receiver:
    image: receiver:latest
    environment:
      Laddr: "127.0.0.1:3000"
    ports:
      - "3000:3000/udp"
    links:
      - sender

  sender:
    image: sender:latest
    environment:
      Laddr: "127.0.0.1:2000"
      Raddr: "127.0.0.1:3000"
    ports:
      - "2000:2000/udp"