[ADD] Halve CI run time by folding certain image steps into a consistent base image

Asmund THORSecrequested to merge
asmund/smoketest-speed into develop

Further CI improvements:

  • Folded repeated apk/apt installs into consistent base image.
  • Switched to pinned image versions in CI usage.

CI runtime was ~50 minutes before, now hovers in the low 20s.

Edited by Heimdall

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