This changes the flutter tool to just try 10 times before giving up when running "flutter upgrade". Infinite retries can hang bots, and really don't provide a lot of help: if we've failed to upgrade for for nearly a minute, trying every five seconds, then something is just not responding.
Also, changed the bot default warning level to "normal" from "all", because the solver messages are VERY verbose: several megs of output for doing packages get on Flutter. "normal" will give warnings, user messages and errors, which should be sufficient to diagnose problems on the bots without spamming the log.
I removed the retrying for building the snapshot on flutter.bat because we don't do that on the other platforms, and because I can't imagine how running it again would give a different answer.
I also fixed a problem in the whitespace detection when no files matched the type of file that it is looking for, and removed the code that waits until failure to print the logs on setup, since reducing the log output made a huge difference.
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.
