prevent docker build from using cache when downloading tinfoilhat

currently, in infrastructure/docker/yoctobuild.dockerfile tinfoilhat and aliensrc_creator are manually downloaded from their repos, but since the corresponding RUN command never changes, docker build command always uses cache even if tinfoilhat has been updated.

This is a known issue in docker, more insights here and here

This MR implements a solution like this

relates to issue #41 (closed)