Add a OpenSUT startup script

[podhrmic]

Fixes #96

EDIT: docker/build-push-action@v6 does not seem to preserve the .git directories in the build context, so it is not possible to run git commands in the COPY-ed repo. Using plain docker build invocation on the other hand does not preserve the cache, but it keeps the git history.

Perhaps a suitable way forward is to run the docker build nightly, and always build it from scratch. After all, at some point we need to show how to build the system from scratch.

[podhrmic]

@spernsteiner this is the MR

[spernsteiner]

The docker image now builds successfully in CI. I'm going to try pulling the image and checking that it includes everything needed to run the OpenSUT, and then I'll merge this PR.

[spernsteiner]

I got the Docker image working locally. I'm now working on writing up documentation on how to start the OpenSUT, run the mission, and read the encrypted logs afterward.

[spernsteiner]

Added instructions. There are a couple TODOs, but I think this is basically done.

[spernsteiner]

There's now only one remaining TODO, which may be better left to a separate PR. The current instructions in OPENSUT_DOCKER.md use the base_single.toml configuration, which has only a single level of VMs: the four guest VMs are each run directly under the Docker container. The TODO is to instead use the "full" OpenSUT configuration, where an emulated pKVM host kernel runs under the Docker container, and the guests in turn run under the host. The base_nested.toml configuration is supposed to implement this, but it's currently out of date (missing some recent changes to the setup) and also needs fixes to support running multiple guests simultaneously.