This problem has been fixed with ivy 7.4. It was related to our debian package and because our docker image is based on the debian package, it was also affected by this issue.
You can now add your extensions in the dropins `dropins` directory and this will work within the first start of the engine. Have a look at our custom valve [docker-sample][1].[docker-sample][1].
[1]: https://github.com/ivy-samples/docker-samples