introduce a builder image

[ynohat]

Capturing @jpayne-procella's idea here for future consideration

I'm hoping that upx gets fixed for aarch64 before it changes names which means that the apk search --no-cache | grep -q ^upx approach might be better than trying to list out all architectures that should have upx - if it's supposed to install and doesn't at least we'll have an error to look at. And worst case upx doesn't get used which doesn't break functionality.

The other other option I was thinking of would be adding a upx-builder image to handle the messiness. Maybe a common-builder which could pull in upx, git, curl, etc. In that case, it could even pull in upx 3.95 from alpine3.11 community repo if upx isn't in the default repo.

Originally posted by @jpayne-procella in #55 (comment)