Add .dockerignore for faster docker build times

On OSX, the entire directory structure, including artifacts, are copied
into the docker machine every docker build.

This eliminates copying some large artifacts, git binary data, etc. to
save a few seconds each build attempt.

.dockerignore

@@ -0,0 +1,8 @@
+# we want to get the git commit SHA but not all the binary repo data
+.git/objects/pack
+bin/
+vagrant/
+docs/
+scripts/gobgp
+*.tar
+*.bz2