CI: Always rebuild test images with --no-cache before container and E2E tests

This ensures that GitHub Actions never reuses outdated Docker layers and that
each test run starts from a fully clean environment. The workflows for
test-container and test-e2e now explicitly invoke:

    distro="${{ matrix.distro }}" make build-no-cache

before executing the actual tests.
This aligns the CI behaviour with local testing, eliminates hidden caching
differences, and guarantees deterministic test results across all distros.

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

.github/workflows/test-container.yml

@@ -19,6 +19,11 @@ jobs:
       - name: Show Docker version
         run: docker version
 
+      - name: Build test image with no cache (${{ matrix.distro }})
+        run: |
+          set -euo pipefail
+          distro="${{ matrix.distro }}" make build-no-cache
+
       - name: Run container tests (${{ matrix.distro }})
         run: |
           set -euo pipefail

.github/workflows/test-e2e.yml

@@ -19,6 +19,11 @@ jobs:
       - name: Show Docker version
         run: docker version
 
+      - name: Build test image with no cache (${{ matrix.distro }})
+        run: |
+          set -euo pipefail
+          distro="${{ matrix.distro }}" make build-no-cache
+
       - name: Run E2E tests via make (${{ matrix.distro }})
         run: |
           set -euo pipefail