Create new repo PandABlocks-devcontainer for docker image

[EmilioPeJu]

Decision: Separate Docker Images for rootfs build and FPGA testing

We will continue using the kas image to build the rootfs without modifications, staying aligned with the official kas workflow and avoiding the introduction of new issues.
In addition, we will introduce a dedicated, lightweight Docker image for running FPGA timing tests and the simulator — intentionally kept smaller than the kas image since it does not need rootfs build tooling.

Proposed approach:

• Pull the existing Docker image from PandABlocks-rootfs (together with the CI configuration).
• Gradually slim it down over time by removing anything unrelated to FPGA testing — starting with all rootfs build dependencies.

[coretl]

After thinking about this a bit more, I think a devcontainer per repo is a better fit. We will use ubuntu-devcontainer as the base for the server repo, probably the FPGA one too, then use kas for meta-panda. Then we just need to add nodejs so we can run mystmd in it like the CI does.