Migrate Travis and Docker build/push to GitHub Workflows

**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...