Since the prefix for shntools is /usr, the default path for headphones is /usr/bin, not /usr/bin/shntool.
This leads to a missing binary error in logs.
```
root@eb24f530c9b0:/$ ls /usr/bin/shn*
/usr/bin/shncat /usr/bin/shncmp /usr/bin/shnconv /usr/bin/shncue /usr/bin/shnfix /usr/bin/shngen /usr/bin/shnhash /usr/bin/shninfo /usr/bin/shnjoin /usr/bin/shnlen /usr/bin/shnpad /usr/bin/shnsplit /usr/bin/shnstrip /usr/bin/shntool /usr/bin/shntrim
```
This change allows using Headphones builtin mechanism to set and use the
listening TCP port.
This is particularly useful when using macvlan networking as docker's -p
option isn't functional in this case.
