This switches docs and gallery build and deployment to use Cirrus CI instead of Travis, reapplying the changes from #19925, but updating the Firebase tokens for upload, and refining docs.sh some to limit retries to five times (instead of indefinitely).
This adds a Docker image for the linux builds, replacing a lot of the setup code with a Docker build.
Added a docker image build step that has the right gcloud credentials in it.
Also, this finally moves the gallery deployment and docs publishing steps to Cirrus. They were dependent upon some environment setup that was a lot easier to do in Docker than in a setup bash script.
