Add support for runArgs devcontainer property

[https://github.com/coder/envbuilder/blob/main/docs/devcontainer-spec-support.md]() does not currently mention the [`runArgs`]() part of the dev container specification:

> An array of [Docker CLI arguments]() that should be used when running the container. Defaults to \[\]. For example, this allows ptrace based debuggers like C++ to work in the container:
"runArgs": \[ "--cap-add=SYS_PTRACE", "--security-opt", "seccomp=unconfined" \] .

Supporting this would make the envbuilder here much more flexible for various use cases where customization of the docker containers is needed.

Start with docs/devcontainer-spec-support.md and compare its supported properties with the linked dev container specification entry for runArgs. Trace how supported properties become Docker run arguments, then add coverage for the documented example and verify that the resulting container receives those arguments.

Written by the indexing model from the issue text.