Migrate CI from Travis → GitHub Actions

[victorlin]

Description of proposed changes

• Migrate .travis.yml contents to a GitHub Actions workflow file.
• Migrate push-from-travis logic into the workflow file.

Related issue(s)

N/A

Testing

See PR checks. However, this does not test the step used to push latest images from master.

Pre-merge

Ensure the following PRs have been approved and are ready for merge:

• Call new GitHub Actions workflow to rebuild images augur#910
• Call new GitHub Actions workflow to rebuild images auspice#1505
• ensure no other references (see query)

Post-merge

• Validate the step used to push latest images from master works as expected.
  • https://github.com/nextstrain/docker-base/runs/6297670211?check_suite_focus=true#step:7:372
  • https://hub.docker.com/layers/nextstrain/base/latest/images/sha256-b5ee1b20ec4902e44d3c5172f06d71f69e5326112d014bb3b83ca882f7f0873f?context=explore
• Merge the PRs approved in pre-merge step.
• Delete environment variables from https://app.travis-ci.com/github/nextstrain/docker-base/settings.
• Remove TRAVIS_AUTH_TOKEN from org Actions secrets.

[tsibley]

Good to start moving this over. I think sooner than later we'll want to move to using a workflow more like https://github.com/nextstrain/ncov-ingest/blob/a590ea875333c04f8c1e02ceaa9cc5689acf2f29/.github/workflows/update-image.yml (with some updated details for different tags and a separate builder image), but that can come in another PR.