Image name inconsistent with index.docker.io

Currently we build tiagoboldt/sharelatex, but the image uploaded to index.docker.io is tiagoboldt/sharelatex-docker:sharelatex

Therefore the `docker run` command mentioned in the README doesn't work with `docker pull...` method.

Either merge this PR, or consider creating the trusted build to be `tiagoboldt/sharelatex:latest`