This PR updates the list of images we build nightly, deprecating
Ubuntu 16.04 and CentOS 8 and adding Ubuntu 20.04, Ubuntu 22.04
and CentOS Stream. It also removes a lot of duplication by generating
the Dockerfiles during the CI workflow and uploading them as artifacts
for later inspection or reuse.
* Unite Dockerfiles - add build/run/push scripts
* update docker documentation
* update .travis.yml
* switch to using a preprocessor on Dockerfiles
* skip building docker images on pull requests
* update files with copyright info
* tweak when travis builds for docker files are done