entrypoint.sh now updates packages on every run

Created by: mor1

Invoking sudo apk -U upgrade -a in entrypoint.sh significantly slows down invocation of dabuild because the updates aren't cached so have to be applied every time.

Better would be to autobuild and update the alpinelinux/docker-abuild images on Hub when packages are updated I think -- is that feasible?

/cc @clandmeter @tcely