json output from k6 on docker

You need to add --user $UID with docker run.

docker run \
  -i \
  -v $(pwd):/ci/ \
  --user $UID \
  loadimpact/k6:latest \
  run \
  --out json=/ci/performance/full.json \
  --summary-export=/ci/performance/summary.json \
  /ci/packages/faas-penguin-local/performance-test.js

And this is actual example:
faas-penguin/config.yml at c31ab0c47feb9c26ed8b3297468568dfbf6f25cd · sanemat/faas-penguin

refs:
Permission denied on k6 docker json output - k6 support - k6 community forum