build-unit-test-docker:eslint: pin to v8.56.0

The new v9.0 release change the API and our existing use of it no longer
works with an error like this:

```
Invalid option '--eslintrc' - perhaps you meant '--ignore'?
```

Until we have some time to sort this out, move us back to the previous
release.

Change-Id: Ieed62b82bb3870c22bb377f562fb75b148395cee
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
1 file changed