Add docker file and image

A new docker file and image is necessary for MICADO because:

• MICADO-WISE is only compatible with Python >=3.6, and the astro/common and conda-builder images are Python 3.5
• It is necessary to pre-install (or at least pre-download) packages in order to make the continuous integration speed up. (This would be relevant for the other images too, but the packages are different.)