Use script dir instead of referencing a relative path for project root (fixes #3)

scripts/build_docker.sh

@@ -1,4 +1,6 @@
 #!/bin/bash
 
-docker build -t xgboost:latest - < ./scripts/Dockerfile-xgboost
-docker build -t xgboost-testing:latest - < ./scripts/Dockerfile-testing
+DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null && pwd )"
+
+docker build -t xgboost:latest - < "$DIR/Dockerfile-xgboost"
+docker build -t xgboost-testing:latest - < "$DIR/Dockerfile-testing"