Observations from 2021.8.1 automatic release

A couple unexpected things happened with the latest automatic release procedure:

- [x] GitHub actions didn't run when the pull request was initially opened (though when I pushed an empty commit, GHA ran as expected). I checked https://www.githubstatus.com/ and there were know reported issues with GitHub actions during this time. xref https://github.com/dask/dask-docker/pull/178
- [ ] Even though the `automerge` label was applied to the PR, it wasn't automatically merged after all CI checks passed. xref https://github.com/dask/dask-docker/pull/178
- [x] Twice as many `build` CI jobs ran on https://github.com/dask/dask-docker/pull/178. I suspect this is because we're triggering on both `pull_request` and `push` events
https://github.com/dask/dask-docker/blob/e82ac06b362319833954ae64687061606f22b924/.github/workflows/build.yml#L3
- [x] The PR which was automatically created updated the `release: XXXX.XX.X` field in `build/docker-compose.yml` but not `.github/workflows/build.yml` (xref https://github.com/dask/dask-docker/pull/179). EDIT: fixed by https://github.com/dask/dask-docker/pull/181
- [x] Images for the new 2021.8.1 release aren't appearing on docker hub (https://hub.docker.com/r/daskdev/dask/tags?page=1&ordering=last_updated) (I _think_ this is related to our autotagging action)