Use hotosm/gh-workflows for container img builds

[spwoodcock]

What type of PR is this? (check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• 📝 Documentation
• 🧑‍💻 Refactor
• ✅ Test
• 🤖 Build or CI
• ❓ Other (please specify)

Describe this PR

@eternaltyro

• This PR uses the hotosm/gh-workflows image build workflow for the backend.
• Everything is preconfigured, including vulnerability scanning, correct tagging, ghcr registry caching, multi-arch builds (AMD64/ARM64).
• The frontend can be built in a similar way, but as the tasking-manager.env is copied into the image, either:
  • The dockerfile would need to be updated to take ENV/ARG variables for everything instead.
  • The workflow would have to be updated to support copying the .env in during the build (no recommended).

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Alternative Approaches Considered

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Review Guide

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Checklist before requesting a review

• 📖 Read the Tasking Manager Contributing Guide: https://github.com/hotosm/tasking-manager/blob/develop/docs/developers/contributing.md
• 📖 Read the HOT Code of Conduct: https://docs.hotosm.org/code-of-conduct
• 👷‍♀️ Create small PRs. In most cases, this will be possible.
• ✅ Provide tests for your changes.
• 📝 Use descriptive commit messages.
• 📗 Update any related documentation and include any relevant screenshots.

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[sonarcloud]

Quality Gate passed

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