Fix so the demo notebook launches on MyBinder

[fomightez]

A user, TSSFL, posted in the Jupyter Discourse forum here seeking help when that they could not launch the CAMB demo notebook via MyBinder.org.

The issue is that there are a couple of ways in which the present Dockerfile in the CAMB repo deviates from the documentation for preparing a Dockerfile and at present this causes the launch to never work.

SOLUTION: A few edits to the Dockerfile were necessary to bring it in line with the directions, as implemented in the fork here. (The Dockerfile specifically is here.) If you use the launch binder badge there, it will use that forked repo as the source and start up. The launched session was verified to be capable of actively running the demo notebook.

I'm not doing this as a Pull Request because I am unsure exactly what image tag you'd specifically like to use in the FROM command in the Dockerfile.

(While you are doing this, you may optionally want to update the launch binder badge to match the newer, better-branded style, as can be seen near the top of the README of the fork here.)

[cmbant]

Thanks, merged and seems to work.