Deploy ocaml-dune/binary-distribution preview

[maiste]

Dune binary distribution

Context

The goal of this PR is to discuss the requirement and work to make https://ocaml-dune/binary-distribution live for the Dune Developer Preview. We stick to this part of the deployment right now. We will open a new issue when we will need to do add the stable version. Let's take the simplest path for now 👌

The goal of the Dune Developer Preview is to provide dune binaries with all the unstable features activated. Thanks to beta testers, we will be able to iterate and polish the features before considering them as stable. It won't influence the opam deployment of Dune, as it would still be done using the regular channel. The unstable binaries are currently produced using GitHub Actions. We will explore other options in the future, but this is not the target of this issue.

Hosting requirements

In the context of the Dune Developer Preview, the requirements are:

•  The website currently hosts on https://dune.ci.dev should be hosted under https://preview.dune.build. The main branch contains what we want to deploy. This is achieved by Add ocaml-dune/binary-distribution #234. It will make the intention of the website and the unstable binary distribution clearer.
• The hosted content (binaries) are currently hosted under https://download.dune.ci.dev. The DNS should change to point it to https://get.dune.build. It will permit to keep the "~S3 bucket" accessible and make the people able to install the latest version of the binaries. They will still be built under GitHub Actions and pushed using ssh as it was done in the past. Only the DNS target change here, but not the content and the build mechanism. I will take care of changing the target in the ocaml-dune/binary-distribution variables.

Conventions

In terms of naming, the repository is named ocaml-dune/binary-distribution. It can be referenced everywhere else as dune-binary-distribution. The docker image can be named ocurrent/dune-binary-distribution:live.