ubuntu-distro: Update Dockerfile

- Clean up apt lists after installing all packages.

Signed-off-by: Chirag Shilwant <c-shilwant@ti.com>

ubuntu-distro/Dockerfile

@@ -4,5 +4,6 @@ LABEL org.opencontainers.image.source https://github.com/cshilwant/docker-images
 RUN export DEBIAN_FRONTEND=noninteractive; apt-get update
 RUN export DEBIAN_FRONTEND=noninteractive; \
          apt-get install -y --no-install-recommends file build-essential autoconf automake bison flex libssl-dev bc u-boot-tools swig python3 python3-pip wget nano device-tree-compiler python3-dev && \
-         pip3 install jsonschema pyelftools PyYAML
+         pip3 install jsonschema pyelftools PyYAML && \
+         rm -rf /var/lib/apt/lists/*