Create a GIF with "boomerang effect" from a video (from mp4, webm, ...) using ffmpeg and bash command line in Linux

Shell scripting question. I'm interested in taking a simple MP4 and obtain a GIF loop with boomerang effect:

• INPUT (mp4, mkv, webm, videos): somebody throwing a ball very distant

• OUTPUT (GIF): somebody throwing a ball very distant and receiving it back, forever

Ideal characteristics of the script:

• suitable for /bin/sh (but /bin/bash is fine too)

• high quality result GIF

• only tools packaged by Debian or Ubuntu in the main repository (e.g. ffmpeg is good)

• absolutely no third party web services

• possibly not consuming an incredible amount of RAM memory and CPU time

I've checked many other questions but seems to do not completely cover my need:

• FFMpeg : Converting a video file to a gif with multiple images from video - about simple video→GIF conversion, not about "boomerang"

• Creating a GIF animation from PNG files while constraining GIF size - about constraining a GIF, not about "boomerang"

• Creating a GIF animation from PNG files while constraining GIF size - about optimising frames, not about "bommerang"

• Creating a GIF animation from PNG files - about creating a PNG→GIF, not about "boomerang"

• Loop MP4 From Converted Animated Gif - about GIF→mp4