devcontainer

Attempt manual docker login when pushing images.

Build LLVM from source as part of the devcontainer Dockerfile, rather than attempting to reuse artifacts.

Revert the OS change, as COS doesn't work on cirun.io. Instead, install the docker-buildx package from apt.

Run Google container-optimized OS on devcontainer runners, which should come with container-related tools pre-installed.

Use the permissions and name when installing docker