coreboot-sdk: Remove unnecessary files

Reduces the size of the Docker image by removing all unnecessary files.

Change-Id: Ib8c658799217c3b6595e3b5fce8f5c8238054c45
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/86617
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

util/docker/coreboot-sdk/Dockerfile

@@ -90,7 +90,8 @@ RUN \
 		xfonts-unifont \
 		xz-utils \
 		zlib1g-dev \
-	&& apt-get clean
+	&& apt-get clean \
+	&& rm -rf /var/lib/apt/lists/*
 
 RUN \
 	cd /tmp && \