🐛 Fix apt mirror URLs in EOL and bootstrap Dockerfiles 🐛

18.04 and 20.04 were pointing to old-releases.ubuntu.com for bionic/focal,
which returns 404s (those releases have been removed from old-releases).
24.10 arm64 was using old-releases.ports.ubuntu.com which doesn't exist —
old-releases.ubuntu.com serves all architectures.
26.04 was trying to pull casjaysdev/ubuntu:26.04 as its base, causing a
chicken-and-egg failure since that image hasn't been built yet.
- Dockerfile.18.04: switch amd64 mirror to archive.ubuntu.com/security.ubuntu.com, arm64 to ports.ubuntu.com/ubuntu-ports
- Dockerfile.20.04: same mirror fix as 18.04 for focal
- Dockerfile.24.10: unify arm64 old-releases to old-releases.ubuntu.com/ubuntu (no ports variant exists)
- Dockerfile.26.04: change PULL_URL from casjaysdev/ubuntu to ubuntu to bootstrap from official base image

Dockerfile.18.04
Dockerfile.20.04
Dockerfile.24.10
Dockerfile.26.04

Dockerfile.26.04

@@ -26,7 +26,7 @@ ARG IMAGE_REPO="casjaysdev/ubuntu"
 ARG IMAGE_VERSION="26.04"
 ARG CONTAINER_VERSION=""
 
-ARG PULL_URL="casjaysdev/ubuntu"
+ARG PULL_URL="ubuntu"
 ARG DISTRO_VERSION="${IMAGE_VERSION}"
 ARG BUILD_VERSION="${BUILD_DATE}"