[DO NOT MERGE] Preliminary Ubuntu 20.04 images for both CPU and GPU as well as docs

[fplk]

• Current image in master will not build at all due to wget https://julialang.org/juliareleases.asc. That will not resolve, since this file was moved to https://julialang.org/assets/juliareleases.asc The PRed images work.
• Currently, there are no container instructions - neither in the Dockerfile nor in the Github readme nor in the Gen docs, so I have added docs to build and run the image
• Images for CPU and GPU
• Updates Julia 0.7 (from 2018) to 1.5.1 (current)
• Updates Ubuntu 16.04 to 20.04, since 16.04 is in ESM - cmp. release cycle whereas 20.04 received full hardware & maintenance updates until 2022, LTS until 4/2025 and ESM until 04/2030
• Tweaks: Updates MAINTAINER tag which is deprecated to LABEL, removes cached files and uses --no-install-recommends for smaller image size, reduces number of layers caused by separate RUN lines in current image
• Forms foundation to derive other Gen project images from (I will soon push POCs for 1 external and 2 internal projects)

We might consider:

• Adding JupyterLab (which I already have in internal image)
• Consolidating this with the gen-quickstart image (which I have also contributed)
• First make sure that internal projects can successfully base their containers on these before merging
• Further investigate issue 311 before merging, since this currently blocks using Gen in containers on macOS
• Could add checksum verification back in like the previous image did

Happy to accommodate change requests - looking forward to feedback.