[Dependabot] groups docker dependencies

[cheina97]

Description

This PR enables grouping for docker dependencies in dependabot PRs

[adamjensenbot]

Hi @cheina97. Thanks for your PR!

I am @adamjensenbot.
You can interact with me issuing a slash command in the first line of a comment.
Currently, I understand the following commands:

• /rebase: Rebase this PR onto the master branch (You can add the option test=true to launch the tests
  when the rebase operation is completed)
• /merge: Merge this PR into the master branch
• /build Build Liqo components
• /test Launch the E2E and Unit tests
• /hold, /unhold Add/remove the hold label to prevent merging with /merge

Make sure this PR appears in the liqo changelog, adding one of the following labels:

• feat: 🚀 New Feature
• fix: 🐛 Bug Fix
• refactor: 🧹 Code Refactoring
• docs: 📝 Documentation
• style: 💄 Code Style
• perf: 🐎 Performance Improvement
• test: ✅ Tests
• chore: 🚚 Dependencies Management
• build: 📦 Builds Management
• ci: 👷 CI/CD
• revert: ⏪ Reverts Previous Changes

[fra98]

/rebase test=true

[fra98]

/merge