It turns out Github Actions have docker pre-installed.
This patch unifies the codepaths that we use to build docker image
locally and to publish it from CI.
As a result, there's no extra `--prepare-context` argument in
`//docs/docker/build.sh` that was used solely on CI.
References #2926
This patch bakes browsers into docker image.
Important observations:
- We now re-build docker image everytime we roll browsers.
- Docker image size almost doubles: from `225MB` to `496MB`.
References #2926