chore: let dependabot update docker

[reneleonhardt]

Changes proposed in this pull request:

• Let Dependabot update Dockerfiles

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• [ ] I've read and followed all steps in the Making a pull request section of the CONTRIBUTING docs.
• [ ] I've updated or added any relevant docstrings following the syntax described in the Writing docstrings section of the CONTRIBUTING docs.
• [ ] If this PR fixes a bug, I've added a test that will fail without my fix.
• [ ] If this PR adds a new feature, I've added tests that sufficiently cover my new functionality.

[reneleonhardt]

I saw that your Dockerfile is outdated, it's easier to let Dependabot update them too.