ci/docker: unify image build logic and run virgin tests across all distros

Refactor Dockerfile into multi-stage virgin/full targets and introduce a single
flag-based image build script. Standardize image naming, remove redundant build
scripts, and update Makefile targets accordingly. CI workflows now build missing
virgin images and run root and user tests consistently across all supported
distributions.

https://chatgpt.com/share/693c29d9-9b28-800f-a549-5661c783d968

Makefile

@@ -57,10 +57,16 @@ setup-nix:
 # Docker build targets (delegated to scripts/build)
 # ------------------------------------------------------------
 build:
-	@bash scripts/build/build-image.sh
+	@bash scripts/build/image.sh --target virgin
+	@bash scripts/build/image.sh
+
+build-missing:
+	@bash scripts/build/image.sh --target virgin --missing
+	@bash scripts/build/image.sh --missing
 
 build-no-cache:
-	@bash scripts/build/build-image-no-cache.sh
+	@bash scripts/build/image.sh --target virgin --no-cache
+	@bash scripts/build/image.sh --no-cache
 
 build-no-cache-all:
 	@set -e; \
@@ -88,13 +94,6 @@ test-env-virtual: build-missing
 test-env-nix: build-missing
 	@bash scripts/test/test-env-nix.sh
 
-
-# ------------------------------------------------------------
-# Build only missing container images
-# ------------------------------------------------------------
-build-missing:
-	@bash scripts/build/build-image-missing.sh
-
 # Combined test target for local + CI (unit + integration + e2e)
 test: test-env-virtual test-unit test-integration test-e2e