Only check out latest files

To build the image. we don't need the whole history.
https://git.wiki.kernel.org/index.php/GitFaq#How_do_I_do_a_quick_clone_without_history_revisions.3F

Docker/README.md

@@ -15,7 +15,7 @@ Simple and fast setup of EOS.IO on Docker is also available.
 ## Build eos image
 
 ```bash
-git clone https://github.com/EOSIO/eos.git --recursive
+git clone https://github.com/EOSIO/eos.git --recursive  --depth 1
 cd eos/Docker
 docker build . -t eosio/eos
 ```