Create base image based on Ubuntu 20.04 LTS

[eschnett]

I need to use several software packages that are either not available in Ubuntu 18.04, or are outdated there. It would be quite convenient for me if the base image was using Ubuntu 20.04LTS instead of 18.04LTS.

For reference, I need these packages which are not available in 18.04:

• cmake >= 3.14
• gcc >= 8
• libopenmpi-dev >= 3
• libpsm2

I can, of course, install these from source, but this is time consuming every time the image is rebuilt.

[ajslone]

@eschnett I have a PR in to add this to our cloudbuild script (#78), and I manually generated cpu images for 20.04 if you'd like to give it a try: gcr.io/blueshift-playground/blueshift:cpu-ubuntu2004-py37 and gcr.io/blueshift-playground/blueshift:cpu-ubuntu2004-py38

[eschnett]

@ajslone I tried the py38 version, and I like it. My Dockerfile is now half the size. Thanks a lot!