That was my plan that I did not mention to you as I was also not able to find php.ini. It seems that the https://github.com/docker-library/owncloud/issues/99 addresses this.
Also, the discussion at https://github.com/docker-library/wordpress/issues/10 might offer a complete solution. If you agree, then you may consider adding the “cleaned out” text from that discussion to the page at https://docs.docker.com/compose/wordpress/