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: https://github.com/docker-library/wordpress/commit/d775299ddae6dafa232166f44b7c03d23ddf7bb6

[alcohol]

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