add copied public to ignore (#4103)

# Summary

When building tests images locally you might create that public folder
as seen here:
https://github.com/10gen/ops-manager-kubernetes/blob/a7fa2fec464b90a0707f8454c92b6c9025ee89cf/pipeline.py#L463

we should ignore that folder as well

## Proof of Work

- build image locally
- git should detect no changes

## Checklist
- [ ] Have you linked a jira ticket and/or is the ticket in the title?
- [ ] Have you checked whether your jira ticket required DOCSP changes?
- [x] Have you checked for release_note changes?

Author: nammn

.gitignore

@@ -49,6 +49,7 @@ docker/mongodb-enterprise-init-database/Dockerfile
 docker/mongodb-enterprise-init-ops-manager/Dockerfile
 docker/mongodb-enterprise-operator/content/mongodb-enterprise-operator.tar
 docker/mongodb-enterprise-tests/helm_chart/
+docker/mongodb-enterprise-tests/public/
 docker/mongodb-enterprise-tests/requirements.txt
 docker/mongodb-enterprise-tests/.flake8
 docker/mongodb-enterprise-tests/.pylintrc