This starts publishing our Docker images using Github Actions:
- on every commit, a new image will be pushed, tagged with a SHA of the
commit, e.g.
`mcr.microsoft.com/playwright:sha-fe997eca88d6de48378b05133dadfe032bd8b0ea`
- on every release, a new image will be pushed, tagged with the release
version, e.g. `mcr.microsoft.com/playwright:v1.2.1`
**Note:** today we don't version our images and instead push them all as
`bionic`. This patch retains the tag - along with `latest`. Both
`bionic` and `latest` tags will point to the latest release.
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