Container failed to start when running as non-root (user 1000:1000) because
copied directories had restrictive 770 permissions, likely due to different
umask in persistent BuildKit. Non-root users couldn't access /usr/lib to
load required dynamic libraries.
Introduces prepper stage using Ubuntu to organize files into layered structure
with explicit 755 directory permissions before copying to scratch image.
Also fixes workflow syntax error and removes docker/** from paths-ignore to
ensure Docker changes trigger CI builds.
Tom Foster
Jade Ellis