Cerberus support

Now, as https://github.com/compiler-explorer/compiler-explorer/pull/5864 have been merged (Thanks!), I want to add support from Cerberus to the main website.

@mattgodbolt wrote: "The process could do with a look at: but basically if you have a system that can install the compiler from a tarball, or a dockerfile that can bulid it for Ubuntu 20 or 22, then we can help :) The compiler needs to be able to be somewhat hermetically built with no external dependencies (or minimal) as it will be run in a very restricted environment. Let me know if I can help more!"

We do not presently have a tarball, but we have a very simple ubuntu-based docker container:

1. [Dockerhub](https://hub.docker.com/repository/docker/vzaliva/cerberus-cheri/general)
2. [Dockerfile](https://github.com/rems-project/cerberus/blob/master/Dockerfile.cheri)

As you can see, the installation is done via OPAM package manger, which is, in turn, provided by Ubuntu (and other distros). Theoretically one can make OPAM to install everything under `/opt` or any other location by providing `--root=/opt/compiler-explorer/.opam` command line option to `opam init` command.

I am volunteering to do what's needed but need some guidance.

Start by reviewing the merged Compiler Explorer PR #5864, then inspect Dockerfile.cheri and the linked Docker Hub image to understand the current Cerberus installation. Confirm what the existing compiler-support process requires for a hermetic Ubuntu image and an installation under a controlled path; done means Cerberus is available on the main website.

Written by the indexing model from the issue text.