Proposal: add flag to skip bind mounts where host folder doesn't exist

When working with dev containers locally (e.g. in Visual Studio Code), it can be productive to mount host folders in the container (e.g. `.azure` to share login credentials into the container).

When using such a dev container in CI, you can run into issues because the host folder doesn't exist (see https://github.com/devcontainers/ci/issues/166).

One way to address this would be to add a `--skip-non-existent-bind-mounts` flag, and have the CLI strip out any bind mounts where the host folder doesn't exist. This flag wouldn't be specified via VS Code (preserving the existing behaviour), but could be specified in CI if desired (e.g. added as an option to https://github.com/devcontainers/ci)