ci(docker): fix repo mount path for pkgmgr as base layer of Infinito.Nexus

Standardize Docker/CI/test environments to mount pkgmgr at /opt/src/pkgmgr.
This makes the layering explicit: pkgmgr is the lower-level foundation used by
Infinito.Nexus.

Infra-only change (Docker, CI, shell scripts). No runtime or Nix semantics changed.

https://chatgpt.com/share/69427fe7-e288-800f-90a4-c1c3c11a8484

scripts/test/test-unit.sh

@@ -6,19 +6,19 @@ echo ">>> Running UNIT tests in ${PKGMGR_DISTRO} container"
 echo "============================================================"
 
 docker run --rm \
-  -v "$(pwd):/src" \
+  -v "$(pwd):/opt/src/pkgmgr" \
   -v "pkgmgr_nix_cache_${PKGMGR_DISTRO}:/root/.cache/nix" \
   -v "pkgmgr_nix_store_${PKGMGR_DISTRO}:/nix" \
-  --workdir /src \
+  --workdir /opt/src/pkgmgr \
   -e REINSTALL_PKGMGR=1 \
   -e TEST_PATTERN="${TEST_PATTERN}" \
   "pkgmgr-${PKGMGR_DISTRO}" \
   bash -lc '
     set -e;
-    git config --global --add safe.directory /src || true;
+    git config --global --add safe.directory /opt/src/pkgmgr || true;
     nix develop .#default --no-write-lock-file -c \
       python3 -m unittest discover \
         -s tests/unit \
-        -t /src \
+        -t /opt/src/pkgmgr \
         -p "$TEST_PATTERN";
   '