Migrate Travis and Docker build/push to GitHub Workflows

[justb4]

Is your feature request related to a problem? Please describe. Travis support will end. Also the CI workflow is slow. Docker build on DockerHub as well, plus the Docker image is always triggered/built even if Travis failed...

Describe the solution you'd like Put all of Travis CI content (.travis file) into .github/workflows, including the Docker build/push to DockerHub.

Describe alternatives you've considered GH Workflows makes us dependent on GitHub. Who knwos what the future may hold. Alternatively we may run Travis or Jenkins ourselves, but that needs to be maintained.

Additional context Most major Open Source geospatial software like GDAL has already migrated to GitHub Workflows...