Add ARCH=armv7l version of iso

[afbjorklund]

The Ubuntu image for armv7 is over 1G, so make a smaller image for emulation.

It is slow, but it boots.

Welcome to Alpine Linux 3.18
Kernel 6.1.34-3-virt on an armv7l (/dev/ttyAMA0)

Not every dependency, such as nerdctl-full, is available for arm-v7. But most are...