ci: rework build step

[phbelitz]

Removed the signing of the build image and creation of SBOM to reduce complexity. Also moved the code of the context and build action into the reusable build workflow.

Checklist

• [x] PR is rebased to/aimed at branch develop
• [x] PR follows Contributing Guide
• [x] Added tests (if necessary)
• [x] Extended README/Documentation (if necessary)
• [x] Adjusted versions of image and Helm chart in Chart.yaml (if necessary)

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.34%. Comparing base (19f4cc7) to head (9825155). Report is 40 commits behind head on develop.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1835 +/- ## =========================================== + Coverage 84.00% 91.34% +7.33% =========================================== Files 42 35 -7 Lines 2939 2530 -409 =========================================== - Hits 2469 2311 -158 + Misses 404 168 -236 + Partials 66 51 -15 ```

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