Fix #170: remove shared variables

[nitschSB]

Hi,
I know the contributing guide says

Open a PR only if the issue is labeled with "Accepting Contributions"

I checked other PRs and think this is an outdated rule.

For me only the maven JibLayerFilterExtension is relevant. Maybe the other extensions contain the same bug.

Before submitting a pull request, please make sure to:

• Identify an existing issue to associate
  with your proposed change, or file a new issue.
• Describe any implementation plans in the issue and wait for a review from the repository maintainers.

Fixes #170 🛠️

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