Updated Builder script

[moos3]

Updated the builder so that it doesn't relay on a pre-existing alpine image. This allows for people to create Alpine linux when they can't use public images.

[ncopa]

I am also working on official upstream rootfs images with the v3.5 release

[andyshinn]

Let's revisit this. Can you give a brief demo of using this and how it might affect our current pipeline? Then I can try it out locally.

From the looks of it, it appears to build a alpine:base first which is then used to build each of the images?

[moos3]

Correct it does build base then builds each version from inside that image.