Fix RPM installation for Fedora/CentOS by enforcing offline local install

This commit improves the robustness of the RPM installation step for
Fedora and CentOS by disabling all remote repositories when installing
the locally built package-manager RPM. This prevents failures caused
by flaky or mismatched upstream metadata (e.g., BaseOS repomd.xml
checksum errors). The installer now:

- uses 'dnf5/dnf/yum --disablerepo=*' for fully offline installation
- falls back to 'rpm -i --nodeps' if the package manager fails
- ensures deterministic CI builds without remote repo dependencies

Reference: Conversation with ChatGPT (2025-12-08). https://chatgpt.com/share/6936ff1e-4adc-800f-b508-b5b90c0ef635

Dockerfile

@@ -152,22 +152,31 @@ RUN set -e; \
         echo 'Running rpmbuild...'; \
         cd /root/rpmbuild/SPECS && rpmbuild -bb package-manager.spec; \
         \
-        echo 'Installing generated RPM...'; \
+        echo 'Installing generated RPM (local, offline)...'; \
         rpm_path=$(find /root/rpmbuild/RPMS -name "package-manager-*.rpm" | head -n1); \
         if [ -z "$rpm_path" ]; then echo 'ERROR: RPM not found!' && exit 1; fi; \
         \
         if command -v dnf5 >/dev/null 2>&1; then \
-            echo 'Using dnf5 to install local RPM...'; \
-            dnf5 install -y "$rpm_path"; \
+            echo 'Using dnf5 to install local RPM (no remote repos)...'; \
+            if ! dnf5 install -y --disablerepo='*' "$rpm_path"; then \
+                echo 'dnf5 failed, falling back to rpm -i --nodeps'; \
+                rpm -i --nodeps "$rpm_path"; \
+            fi; \
         elif command -v dnf >/dev/null 2>&1; then \
-            echo 'Using dnf to install local RPM...'; \
-            dnf install -y "$rpm_path"; \
+            echo 'Using dnf to install local RPM (no remote repos)...'; \
+            if ! dnf install -y --disablerepo='*' "$rpm_path"; then \
+                echo 'dnf failed, falling back to rpm -i --nodeps'; \
+                rpm -i --nodeps "$rpm_path"; \
+            fi; \
         elif command -v yum >/dev/null 2>&1; then \
-            echo 'Using yum to install local RPM...'; \
-            yum localinstall -y "$rpm_path"; \
+            echo 'Using yum to install local RPM (no remote repos)...'; \
+            if ! yum localinstall -y --disablerepo='*' "$rpm_path"; then \
+                echo 'yum failed, falling back to rpm -i --nodeps'; \
+                rpm -i --nodeps "$rpm_path"; \
+            fi; \
         else \
-            echo 'No dnf/dnf5/yum found, falling back to rpm -i (no deps)...'; \
-            rpm -i "$rpm_path"; \
+            echo 'No dnf/dnf5/yum found, falling back to rpm -i --nodeps...'; \
+            rpm -i --nodeps "$rpm_path"; \
         fi; \
         \
         rm -rf "/tmp/$srcdir"; \