Port features.json properties for most common runtime arguments

As a part of development for dev container features (#60), we introduced properties for the most common runtime arguments used in `devcontainer.json`'s `runArgs` property. The great part about this implementation is that it worked with Docker Compose as well and is abstracted enough to work with other potential orchestrators or container runtimes down the road. It would be awesome to bring these to `devcontainer.json` as well.

Specifically:

- `privileged`
- `capAdd`
- `securityOpt`
- `init`
- `mounts` in a revised form that is not quite so engine specific
- `entrypoint` (which would be useful with the proposed `overrideEntrypoint` from https://github.com/microsoft/vscode-remote-release/issues/3614#issuecomment-724125719)

We could also bring `containerEnv` into Docker Compose as it is in `features.json`.

Ideally we'd continue this trend whenever a new property along these lines is added to `features.json` (and vice versa).