Run from pre-built image instead of Dockerfile

[Johennes]

Is your feature request related to a problem? Please describe.

The action currently uses image: 'Dockerfile' which means the Docker image is rebuilt any time the action is run. This takes about a minute which for many use cases is far greater than the time needed to actually run the image.

Describe the solution you'd like

Provide a pre-built image and use image: 'docker://...' to speed up the action execution and avoid needlessly rebuilding the image.

Describe alternatives you've considered

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Additional context

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