Add flag to always update branch from origin

[KludgeKML]

If you just want the latest version of whatever branch it's annoying to have the build keep stopping to prompt you, so now setting GOVUK_DOCKER_UPDATE_BRANCH=always (either permanently in your environment or temporarily at the point you call make) will always update from origin whatever branch you're on without bothering you.

[ollietreend]

Great work @KludgeKML! This very feature was on a personal to-do list of mine – so it's great to see it added 🚀

[KludgeKML]

Thanks! If you have time, one improvement I think would be to add an option to the menus so you can move to always updating even if you forget the env var. But I'll get to that eventually.