Refactor CI distro handling and container build scripts

- Introduce a GitHub Actions matrix for `test-container` and `test-e2e`
  to run against arch, debian, ubuntu, fedora, and centos
- Run unit and integration tests only in the Arch container by passing
  `distro="arch"` via make in the corresponding workflows
- Replace the global DISTROS loop with a single `distro` variable in
  the Makefile, defaulting to `arch`, and export it for all scripts
- Update build scripts (build-image, build-image-no-cache, build-image-missing)
  to build images for the selected distro only
- Simplify test-container script to validate a single distro image using
  the `distro` environment variable
- Simplify E2E, unit, and integration test scripts to run against a
  single distro container instead of iterating over all distros

https://chatgpt.com/share/693acbba-9e30-800f-94fb-fea4489e9078

Makefile

@@ -2,11 +2,15 @@
         test build build-no-cache test-unit test-e2e test-integration \
         test-container
 
+# Distro
+# Options: arch debian ubuntu fedora centos
+distro ?= arch
+export distro
+
 # ------------------------------------------------------------
-# Distro list and base images
+# Base images
 # (kept for documentation/reference; actual build logic is in scripts/build)
 # ------------------------------------------------------------
-DISTROS 		  := arch debian ubuntu fedora centos
 BASE_IMAGE_ARCH   := archlinux:latest
 BASE_IMAGE_DEBIAN := debian:stable-slim
 BASE_IMAGE_UBUNTU := ubuntu:latest
@@ -14,7 +18,6 @@ BASE_IMAGE_FEDORA := fedora:latest
 BASE_IMAGE_CENTOS := quay.io/centos/centos:stream9
 
 # Make them available in scripts
-export DISTROS
 export BASE_IMAGE_ARCH  
 export BASE_IMAGE_DEBIAN
 export BASE_IMAGE_UBUNTU