Fix Dockerfile to properly configure source repositories

- Use .sources format instead of sources.list
- Add both deb and deb-src types to enable apt source command
- This fixes the CI build failure where apt source ostree was failing

Dockerfile

@@ -5,6 +5,13 @@ ENV DEBIAN_FRONTEND=noninteractive
 ENV LANG=C.UTF-8
 ENV LC_ALL=C.UTF-8
 
+# Add source repositories
+RUN echo "Types: deb deb-src" > /etc/apt/sources.list.d/debian.sources && \
+    echo "URIs: http://deb.debian.org/debian" >> /etc/apt/sources.list.d/debian.sources && \
+    echo "Suites: unstable" >> /etc/apt/sources.list.d/debian.sources && \
+    echo "Components: main" >> /etc/apt/sources.list.d/debian.sources && \
+    echo "Signed-By: /usr/share/keyrings/debian-archive-keyring.gpg" >> /etc/apt/sources.list.d/debian.sources
+
 # Install build dependencies for OSTree
 RUN apt update && \
     apt install -y \