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>
diff --git a/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index a34552c..ca93b0f 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -870,7 +870,7 @@
         requests
 
 RUN npm install -g \
-        eslint@latest eslint-plugin-json@latest \
+        eslint@v8.56.0 eslint-plugin-json@latest \
         markdownlint-cli@latest \
         prettier@latest
 """