Mount custom options as volume, so ckan config is updated on `fig restart ckan`

At the moment, custom_options.ini is copied during the image build. This makes it static in between `fig build`s. If the user changes the options, they have to do a `fig build` or `fig up` for the changes to take effect.

This pull request makes the file a volume, so if it is changed the user can do a `fig restart ckan`, which is much quicker.

The pull request also makes the volume paths relative to the source folder so `fig up` works out of the box. This also makes contributions back to the project easier as people don't have to remember to remove their hardcoded paths.