Attempt to fix a transient packaging error

The "Sum hash mismatch" error is happening lately when updating
the lists. This tries to workaround that with an extra `apt clean`.

dev-tools/packer/docker/xgo-image/base/Dockerfile

@@ -19,7 +19,10 @@ RUN chmod +x $FETCH
 
 
 # Make sure apt-get is up to date and dependent packages are installed
+# XXX: The first line is a workaround for the "Sum hash mismatch" error, from here:
+# https://askubuntu.com/questions/760574/sudo-apt-get-update-failes-due-to-hash-sum-mismatch
 RUN \
+  apt-get clean && \
   apt-get update && \
   apt-get install -y automake autogen build-essential ca-certificates           \
     gcc-arm-linux-gnueabi g++-arm-linux-gnueabi libc6-dev-armel-cross           \