This docker example is using a standard php image, but I've got another repository (https://github.com/wardviaene/docker-php7) that you can use to build a custom imaged based on the php images. You can then change the json file to point to the built image. You'll need something like docker hub builds (https://docs.docker.com/docker-hub/builds/) to build the dockerfile and host the images.
Hi,
This docker example is using a standard php image, but I've got another repository (https://github.com/wardviaene/docker-php7) that you can use to build a custom imaged based on the php images. You can then change the json file to point to the built image. You'll need something like docker hub builds (https://docs.docker.com/docker-hub/builds/) to build the dockerfile and host the images.