`runArgs` in Dev Containers image isn't shared to `.devcontainers.json` workspace

The issue names custom-image and workspace devcontainer.json configurations but no source files. Start by tracing how runArgs are read and combined when a workspace uses an image, then compare the Docker inspect PidMode and NetworkMode results. Done means runArgs supplied by the custom image are honored by the consuming workspace without repeating them.

Hi, I tried to use runArgs in Dev Containers custom image but this field isn't shared to .devcontainers.json workspace

• Logs :

1. when runArgs is set in Dev Containers custom image :

$ docker inspect f | grep PidMode
            "PidMode": "",    # pid isn't set to host

$ docker inspect f | grep NetworkMode
            "NetworkMode": "bridge",    # network isn't set to host

2. when runArgs is set in Dev Containers workspace that use the custom image :

$ docker inspect f | grep PidMode
            "PidMode": "host",    # pid is set to host

$ docker inspect f | grep NetworkMode
            "NetworkMode": "host",    # network is set to host

Steps to Reproduce :

1. when runArgs is set in Dev Containers custom image :

// custom image configuration
{
	"name": "Custom image",
	"build": {
		"dockerfile": "Dockerfile",
		"args": {
			"devcontainercli": "true"
		}
	},
	"runArgs": [
		"--network=host",
		"--pid=host"
	],
	"capAdd": ["ALL"],
	"securityOpt": ["seccomp=unconfined"],
	"privileged": true,
        [...]
}

// devcontainer.json workspace configuration
{
	"name": "Dev Containers demo",
	"image": "registry.example.com/example/custom-image:version",
	[...]
}

2. when runArgs is set in Dev Containers workspace that use the custom image :

// custom image configuration
{
	"name": "Custom image",
	"build": {
		"dockerfile": "Dockerfile",
		"args": {
			"devcontainercli": "true"
		}
	},
	"capAdd": ["ALL"],
	"securityOpt": ["seccomp=unconfined"],
	"privileged": true,
        [...]
}

// devcontainer.json workspace configuration
{
	"name": "Dev Containers demo",
	"image": "registry.example.com/example/custom-image:version",
	"runArgs": [
		"--network=host",
		"--pid=host"
	],
	[...]
}

Thank you !