🐛 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.18.04

@@ -84,7 +84,7 @@ RUN set -e; \
   echo "$LANG UTF-8" >"/etc/locale.gen"; \
   echo 'export DEBIAN_FRONTEND="'${DEBIAN_FRONTEND}'"' >"/etc/profile.d/apt.sh" && chmod 755 "/etc/profile.d/apt.sh"; \
   arch="$(dpkg --print-architecture)"; \
-  case "$arch" in amd64|i386) ubuntu_mirror="http://old-releases.ubuntu.com/ubuntu"; ubuntu_security="$ubuntu_mirror" ;; *) ubuntu_mirror="http://old-releases.ports.ubuntu.com/ubuntu-ports"; ubuntu_security="$ubuntu_mirror" ;; esac; \
+  case "$arch" in amd64|i386) ubuntu_mirror="http://archive.ubuntu.com/ubuntu"; ubuntu_security="http://security.ubuntu.com/ubuntu" ;; *) ubuntu_mirror="http://ports.ubuntu.com/ubuntu-ports"; ubuntu_security="$ubuntu_mirror" ;; esac; \
   printf '%s\n' \
     "deb ${ubuntu_mirror} bionic main restricted universe multiverse" \
     "deb ${ubuntu_mirror} bionic-updates main restricted universe multiverse" \