Add new runtime environment variable to dgoss to copy additional files

[dklimpel]

Fixes:

• dgoss: Copy multiple files with GOSS_FILES_PATH #896

Checklist

• make test-all (UNIX) passes. CI will also test this
• unit and/or integration tests are included (if applicable)
• documentation is changed or added (if applicable)

Description of change

Allows you to use or copy additional files in the container with the help of new env var GOSS_ADDITIONAL_COPY_PATH in dgoss in order to set up tests hierarchically with gossfile.

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📚 Documentation preview 📚: https://goss--898.org.readthedocs.build/en/898/