CI: Add debugging instrumentation to identify container build/run anomalies

- Added `git rev-parse HEAD` to test-container workflow to confirm the exact
  commit SHA used during CI runs.
- Updated Dockerfile to print BASE_IMAGE and OS release information during
  build for better reproducibility diagnostics.
- Extended test-container script to dump the first 40 lines of
  `docker image inspect` output, allowing verification of the image ID,
  creation time, and applied build args.

These additions help trace discrepancies between local builds and GitHub
Actions, ensuring we can detect mismatches in commit SHA, base image,
or container metadata.

https://chatgpt.com/share/693ae07a-8c58-800f-88e6-254cdb00b676

.github/workflows/test-container.yml

@@ -16,6 +16,9 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
+      - name: Show commit SHA
+        run: git rev-parse HEAD
+
       - name: Show Docker version
         run: docker version
 

Dockerfile

@@ -1,9 +1,12 @@
 # ------------------------------------------------------------
 # Base image selector — overridden by Makefile
 # ------------------------------------------------------------
-ARG BASE_IMAGE=archlinux:latest
+ARG BASE_IMAGE
 FROM ${BASE_IMAGE}
 
+RUN echo "BASE_IMAGE=${BASE_IMAGE}" && \
+    cat /etc/os-release || true
+
 # ------------------------------------------------------------
 # Nix environment defaults
 #

scripts/test/test-container.sh

@@ -7,6 +7,8 @@ echo
 echo "------------------------------------------------------------"
 echo ">>> Testing container: $IMAGE"
 echo "------------------------------------------------------------"
+echo "[test-container] Inspect image metadata:"
+docker image inspect "$IMAGE" | sed -n '1,40p'
 
 echo "[test-container] Running: docker run --rm --entrypoint pkgmgr $IMAGE --help"
 echo