Add an "update.sh"

[J0WI]

@docker-library-bot can run a update.sh script for you to update the COMPOSER_VERSION automatically. Just add it to the team and open a corresponding PR here: https://github.com/docker-library/oi-janky-groovy/blob/master/update.sh/vars.groovy

See e.g. here: docker-library/wordpress@d775299

[alcohol]

Thanks, I'll look into that when I have time.

[alcohol]

@inverse either this or your solution; automation in general is welcome. Of course there will be the future scenario where LTS becomes a completely new major.minor and/or latest a new major, which might be more difficult to take into account (or that just remains a manual process/change).