debug: output container ls to stdout

This will let us know if other containers are somehow leaking during test runs...

tests/unit/test_helper.bash

@@ -19,6 +19,7 @@ SKIPPED_TEST_ERR_MSG="previous test failed! skipping remaining tests..."
 # skips remaining tests on first failure
 global_setup() {
   [[ ! -f "${BATS_PARENT_TMPNAME}.skip" ]] || skip "$SKIPPED_TEST_ERR_MSG"
+  docker container ls
 }
 
 global_teardown() {