fix: add references to BUILDKIT_HOST for setup and fix tests

tests/unit/builder-railpack.bats

@@ -4,6 +4,14 @@ load test_helper
 
 setup_file() {
   install_railpack
+  docker run --rm --privileged -d --name buildkit moby/buildkit:latest
+  touch /etc/default/dokku
+  sudo tee -a /etc/default/dokku <<<"export BUILDKIT_HOST='docker-container://buildkit'"
+}
+
+teardown_file() {
+  docker container stop buildkit || true
+  docker container rm buildkit || true
 }
 
 setup() {