Images without CMD and/or ENTRYPOINT panic

Expected Behavior

$ docker-slim build --target myimage:without-cmd --cmd 'mycmd --arg1 --arg2'

Does not panic. If a --cmd is supplied, it should satisfy the Inspector, otherwise, a sensible error should be produced.

Actual Behavior

docker-slim: message='join the Gitter channel to ask questions or to share your feedback' info='https://gitter.im/docker-slim/community'
docker-slim: message='join the Discord server to ask questions or to share your feedback' info='https://discord.gg/9tDyxYS'
docker-slim[build]: info=http.probe message='using default probe'
docker-slim[build]: state=started
docker-slim[build]: info=params target=myimage:without-cmd continue.mode=probe rt.as.user=true keep.perms=true
docker-slim[build]: state=image.inspection.start
docker-slim[build]: info=image id=sha256: deadbeeffeed5ba1db0b size.bytes=253392289 size.human=1337 MB
docker-slim[build]: info=image.stack index=0 name='myimage:without-cmd' id='sha256:deadbeeffeed5ba1db0b'
docker-slim[build]: info=image.exposed_ports list='8080'
docker-slim[build]: state=image.inspection.done
docker-slim[build]: state=container.inspection.start
docker-slim[build]: info=container status=created name=dockerslimk_36619_20201230105618 id=deadbeeffeed5ba1db0b
panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
github.com/docker-slim/docker-slim/internal/app/master/inspectors/container.(*Inspector).RunContainer(0xc000166000, 0x4, 0xc00026f960)
	github.com/docker-slim/docker-slim/internal/app/master/inspectors/container/container_inspector.go:556 +0x46b1
github.com/docker-slim/docker-slim/internal/app/master/commands/build.OnCommand(0xc0004080f0, 0x7ffeefbffa2b, 0x18, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x1, ...)
	github.com/docker-slim/docker-slim/internal/app/master/commands/build/handler.go:284 +0x172c
github.com/docker-slim/docker-slim/internal/app/master/commands/build.glob..func1(0xc0003c6580, 0x0, 0xc000249a60)
	github.com/docker-slim/docker-slim/internal/app/master/commands/build/cli.go:361 +0x1eb1
github.com/urfave/cli.HandleAction(0x184a960, 0x19ce010, 0xc0003c6580, 0xc0003c6580, 0x0)
	github.com/urfave/cli@v1.22.4/app.go:526 +0x11a
github.com/urfave/cli.Command.Run(0x198cd0e, 0x5, 0x0, 0x0, 0x214f5a0, 0x1, 0x1, 0x19c2585, 0x6c, 0x0, ...)
	github.com/urfave/cli@v1.22.4/command.go:174 +0x57a
github.com/urfave/cli.(*App).Run(0xc0003ce380, 0xc0001a2000, 0xe, 0xe, 0x0, 0x0)
	github.com/urfave/cli@v1.22.4/app.go:279 +0x7c7
github.com/docker-slim/docker-slim/internal/app/master.Run()
	github.com/docker-slim/docker-slim/internal/app/master/app.go:15 +0x54
main.main()
	github.com/docker-slim/docker-slim/cmd/docker-slim/main.go:8 +0x20

Steps to Reproduce the Problem

1. docker pull k8s.gcr.io/etcd:3.4.13-0
2. docker-slim build --target k8s.gcr.io/etcd:3.4.13-0 --cmd 'etcd'
3. Secure helmet, follow duck-and-cover procedure to prevent injury

It appears that images without ENTRYPOINT/CMD cause a panic. I am not sure if it is just CMD that is empty, or CMD+ENTRYPOINT, but this shouldn't result in a panic.

Checking docker inspect shows

[
  {
    # ... 
    "Config" : {
      "Entrypoint": null,
      "Cmd": null,
      # ...
    },
    "ContainerConfig" : {
      "Entrypoint": null,
      "Cmd": [
        "/bin/sh",
        "-c",
        "#(nop) COPY multi:a1881dd50cdbd92225791143eb662674b0a4155ae2577453cd6fae7dab43f859 in /usr/local/bin/ "
      ],
      # ...  
    }
]

Specifications

• Version: 1.33.0
• Platform: darwin

Reproduce the failure with an image lacking CMD and ENTRYPOINT, then inspect internal/app/master/inspectors/container/container_inspector.go around line 556, where the stack trace points. Verify that a supplied --cmd is accepted by the Inspector and that images without a command produce no panic, using the reported docker-slim build scenario as the completion check.