add DUNECI_DOCKER_CACHE variable to disable Docker's cache

This is useful for the weekly automatic rebuild of the images.  These
should always rebuild the images so updated packages from Debian get
included.

bin/duneci-build-docker-image

@@ -19,6 +19,10 @@ for var in ftp_proxy FTP_PROXY http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_
   fi
 done
 
+if [[ "${DUNECI_DOCKER_CACHE:-1}" = 0 ]]; then
+  docker_opts+=(--no-cache)
+fi
+
 if [[ $# -lt 2 ]]; then
   usage
 fi