fix dependabot docker updater

[aleoli]

Description

This PR fixes the Dependabot Docker updater to use all the Dockerfiles in the build directory

[adamjensenbot]

Hi @aleoli. Thanks for your PR!

I am @adamjensenbot.
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Make sure this PR appears in the liqo changelog, adding one of the following labels:

• kind/breaking: 💥 Breaking Change
• kind/feature: 🚀 New Feature
• kind/bug: 🐛 Bug Fix
• kind/cleanup: 🧹 Code Refactoring
• kind/docs: 📝 Documentation

[aleoli]

/rebase test=true

[aleoli]

/merge