To finish the creation of the Let's Encrypt certificate, which will
allows us to use https://docs.buildstream.build/ for accessing the
documentation.

ci: add ACME challenge

See merge request !831

Unsupported sandbox config (UID/GID) is now reported by the element.

On systems without user namespace support, elements with unsupported
sandbox config (UID/GID) are now individually tainted, which disables
artifact push.

The artifact cache is no longer platform-specific.

Now that the platform is independent of the context, explicit
instantiation is no longer required. This avoids issues with platform
instances used across test cases with mismatching context.