Intermittent integration tests failure

[PatStLouis]

Under some conditions, tests fail for trying to recreate an existing image. Might be a race condition and lack of cleanup between test runs.

Sample failed run:
https://github.com/openwallet-foundation/acapy-plugins/actions/runs/22196117682/job/64196752242?pr=2739

Consistent message:

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 > [alice] exporting to image:
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target alice: failed to solve: image "docker.io/library/plugin-image:latest": already exists