Add script to update Transmission peer port

[juanlufont]

The script transmission-port-update.sh updates the peer port used by the torrent client Transmission using its API. The peer port is the port used and announced by Transmission to accept incoming connections. You can use this script to set a VPN forwarded port as the current listening port for Transmission.

The script uses only shell scripting and can run out of the box within the current gluetun docker image, it only relies on the command wget, which is already included by default in the image.

[ku1ik]

I've used this script and it worked on the first try. Thank you!

[ku1ik]

Just reporting that I've been using this for over a month and it's been working flawlessly 👌

[qdm12]

Thanks for the review of the PR (and the wiki PR 👍 ) Just a few minor things to do if possible 🙏 thanks!

[qdm12]

nit just have this as --quiet directly without a variable, to reduce the "API surface" of the script. One can always ask if they need different wget options, and we can add it as a flag.

[juanlufont]

I have moved --quiet flag out of WGET_OPTS but I am keeping the variable to store there, if needed, the API credentials and some extra flags they require. Is this ok?

[iridial]

I did some tests in k3s with gluetun v3.40 running as initContainer for transmission, and using VPN_PORT_FORWARDING_UP_COMMAND as trigger to call the script using: '/bin/sh -c "/scripts/transmission-port-update.sh --port {{PORTS}}"'.
The transmission container takes more time to be available to receive the port forward update request, and this leads the initial call of the script to fail.
I suggest to include an initial check if transmission api service is available, and if not, retry once more after configurable timeout with /bin/sleep <value>.