Change docker base image to Surnet/docker-wkhtmltopdf to support wkhtmltopdf under alpine 3.16

Use https://hub.docker.com/r/surnet/alpine-wkhtmltopdf as a base image for docker, as discussed in https://gitter.im/eikek/docspell.

TODO:

- [ ] Update `FROM` in `joex.dockerfile` in `docker/dockerfiles` to the new base image and build it to check if there are any errors.
~~Update `FROM` in `restserver.dockerfile` in `docker/dockerfiles` to the new base image and build it to check if there are any errors.~~

Is there anything else we need to do to check if everything works with the new base image? Any test to run?