Use a Dockerfile.template to generate both php variant

[madmath03]

There isn't many differences between the apache and fpm version, and the entrypoint is identical.

Using a Dockerfile.template with an update.sh script to generate / update the variants would make maintenance much easier.

For reference:

• https://github.com/nextcloud/docker/blob/master/update.sh
• https://github.com/nextcloud/docker/blob/master/Dockerfile-debian.template

[J0WI]

See #20. Once https://github.com/roundcube/roundcubemail/issues/5827 @docker-library-bot (needs write access) can update the images automatically.

[madmath03]

Reopen until #55 merged.