feat(dependabot): Add dependabot config for dockerfiles and github-actions
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# What does this PR do ?

This change enabled dependabot to automatically generate PRs for updated images in Dockerfiles or updated github-action versions.  This will help the maintainers keep dependencies up to date.

**Collection**: N/A

# Changelog 
- feat(dependabot): Add dependabot config for dockerfiles and github-actions

# Usage
* N/A

# Jenkins CI
To run Jenkins, a NeMo User with write access must comment `jenkins` on the PR.

# Before your PR is "Ready for review"
**Pre checks**:
- [ x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] Did you write any new necessary tests?
- [ ] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [x ] New Feature
- [ ] Bugfix
- [ ] Documentation

## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.
