Add .gitkeep file to docker ignore to prevent -dirty version

[the-glu]

This PR fix #1431 , applying it suggestion to "unignore" the .gitkeep file in build/dev/startup/coverdata/`.

Tested locally, -dirty tag is not present anymore:

docker run -it docker.io/interuss-local/dss core-service
{"level":"info","ts":"2026-04-15T06:54:24.582Z","caller":"core-service/main.go:147","msg":"version","address":":8080","version":"0.0.0+54668503"}
(...)