Add Docker entrypoint script

Author: midwire

docker-entrypoint.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+args=(--work-dir /output --generate-files --clobber)
+
+if [ -n "${COUNTRY:-}" ]; then
+  args+=(--country "$COUNTRY")
+fi
+
+exec bundle exec free_zipcode_data "${args[@]}"