Restore website-builder Dockerfile or use devcontainer

[adleong]

Most of the Github Actions in the website repo use a docker image called buoyantio/website-builder:v1.3.3. However, the Dockerfile that builds this image was removed in https://github.com/linkerd/website/pull/1465

This means that the make docker-build make target is broken and the image used in CI is not reproducible.

We should either restore this Dockerfile, or ideally, just switch to using the Devcontainer in the CI actions.