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

scripts/build/build-image-missing.sh

@@ -4,32 +4,21 @@ set -euo pipefail
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 source "${SCRIPT_DIR}/resolve-base-image.sh"
 
-echo "============================================================"
-echo ">>> Building ONLY missing container images"
-echo "============================================================"
+IMAGE="package-manager-test-$distro"
+BASE_IMAGE="$(resolve_base_image "$distro")"
 
-for distro in $DISTROS; do
-    IMAGE="package-manager-test-$distro"
-    BASE_IMAGE="$(resolve_base_image "$distro")"
-
-    if docker image inspect "$IMAGE" >/dev/null 2>&1; then
-        echo "[build-missing] Image already exists: $IMAGE (skipping)"
-        continue
-    fi
-
-    echo
-    echo "------------------------------------------------------------"
-    echo "[build-missing] Building missing image: $IMAGE"
-    echo "BASE_IMAGE = $BASE_IMAGE"
-    echo "------------------------------------------------------------"
-
-    docker build \
-        --build-arg BASE_IMAGE="$BASE_IMAGE" \
-        -t "$IMAGE" \
-        .
-done
+if docker image inspect "$IMAGE" >/dev/null 2>&1; then
+    echo "[build-missing] Image already exists: $IMAGE (skipping)"
+    exit 0
+fi
 
 echo
-echo "============================================================"
-echo ">>> build-missing: Done"
-echo "============================================================"
+echo "------------------------------------------------------------"
+echo "[build-missing] Building missing image: $IMAGE"
+echo "BASE_IMAGE = $BASE_IMAGE"
+echo "------------------------------------------------------------"
+
+docker build \
+    --build-arg BASE_IMAGE="$BASE_IMAGE" \
+    -t "$IMAGE" \
+    .

scripts/build/build-image-no-cache.sh

@@ -4,14 +4,12 @@ set -euo pipefail
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 source "${SCRIPT_DIR}/resolve-base-image.sh"
 
-for distro in $DISTROS; do
-  base_image="$(resolve_base_image "$distro")"
+base_image="$(resolve_base_image "$distro")"
 
-  echo ">>> Building test image for distro '$distro' with NO CACHE (BASE_IMAGE=$base_image)..."
+echo ">>> Building test image for distro '$distro' with NO CACHE (BASE_IMAGE=$base_image)..."
 
-  docker build \
-    --no-cache \
-    --build-arg BASE_IMAGE="$base_image" \
-    -t "package-manager-test-$distro" \
-    .
-done
+docker build \
+  --no-cache \
+  --build-arg BASE_IMAGE="$base_image" \
+  -t "package-manager-test-$distro" \
+  .

scripts/build/build-image.sh

@@ -4,13 +4,11 @@ set -euo pipefail
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 source "${SCRIPT_DIR}/resolve-base-image.sh"
 
-for distro in $DISTROS; do
-  base_image="$(resolve_base_image "$distro")"
+base_image="$(resolve_base_image "$distro")"
 
-  echo ">>> Building test image for distro '$distro' (BASE_IMAGE=$base_image)..."
+echo ">>> Building test image for distro '$distro' (BASE_IMAGE=$base_image)..."
 
-  docker build \
-    --build-arg BASE_IMAGE="$base_image" \
-    -t "package-manager-test-$distro" \
-    .
-done
+docker build \
+  --build-arg BASE_IMAGE="$base_image" \
+  -t "package-manager-test-$distro" \
+  .