Shellcheck: issues when passing glob for dir

## Affected builder image

`gcr.io/cloud-builders-community/shellcheck`

## Expected Behavior

```shell
shellcheck ./**/*.sh
```

This should run `shellcheck` against all `.sh` scripts within the repo.

## Actual Behavior

It throws an error

```
Step #1 - "Shellcheck": ./**/*.sh: ./**/*.sh: openBinaryFile: does not exist (No such file or directory)
```

## Steps to Reproduce the Problem

1. use `./**/*.sh` as an `arg` in your `cloudbuild.yaml` for the `shellcheck` builder

## Additional Info

Even running the same param with `bash -c "shellcheck ./**/*.sh` is inconsistent.

The test and usage of the `shellcheck` builder should be updated to cover this scenario as it would be quite common