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

[dacog]

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?

[eikek]

I wouldn't change the restserver image. It works and doesn't require wkhtmltopdf, only joex needs this.