Interference of Docker CMD with su: works with su-exec but not with su?
wrong argument order to su

$ZOO_USER needs to be the first argument to su with the "$@" coming afterwards.

su -c "$0" "$ZOO_USER" "$@"


It is ok for the options to come before the username.  It is a good idea to keep them in double quotes so that there aren't issues if someone accidentally puts spaces into it.

Your error message reinforces that this is an order of arguments problem.

even better

Based on comments this would be even better:

su -c "$0" "$ZOO_USER" -- "$@"


su man page excerpt


  Additional arguments may be provided after the username, in which case they are supplied to the user's login shell. In particular, an argument of
  -c will cause the next argument to be treated as a command by most command interpreters. The command will be executed by the shell specified in /etc/passwd for the target user.
  
  You can use the -- argument to separate su options from the arguments supplied to the shell.


busybox docs excerpt:

In a comment the OP asked about why this wouldn't work in Alpine Linux.  Since alpine is based on busybox I looked up the busybox docs. 
 According to the docs busybox doesn't support sending additional arguments to su:

su
su [OPTIONS] [-] [username]

Change user id or become root

Options:

        -p, -m  Preserve environment
        -c CMD  Command to pass to 'sh -c'
        -s SH   Shell to use instead of default shell