Check git state in Dockerfile to catch .dockerignore bugs

scripts/docker-git-state-check.sh

@@ -0,0 +1,12 @@
+set -e
+
+if [ -z "$(git status --porcelain)" ]; then
+  echo "PASSED: Working directory clean"
+else
+  echo "FAILED: Working directory not clean."
+  echo "This is likely because the .dockerignore file has removed something checked into git."
+  echo "Add the missing files listed below to the .dockerignore to fix this issue:"
+  echo "$(git status)"
+  exit 1
+fi
+