fix: Small fixes resolving naming and nil-pointers

[nderjung]

Prerequisite checklist

• Read the contribution guidelines regarding submitting new changes to the project;
• Tested your changes against relevant architectures and platforms;
• Ran make fmt on your commit series before opening this PR;
• Updated relevant documentation.

Description of changes

GitHub-Fixes: #1551

[LucaSeri]

Hi @nderjung. Thanks for this. kraft pkg works as expected now.
One question about the runtime flag: is the following setup meant to work?

Kraftfile
spec: v0.6

rootfs: ./Dockerfile
  
cmd: ["/server"]

then:

kraft run --runtime unikraft.org/base:latest .

I get the following errors:

 E  could not determine how to run provided input: first positional argument is a directory: .
first positional argument is a directory: /home/kero/unikraft-work/catalog/examples/http-go1.21
cannot run project build without unikraft
cannot run project without runtime directive
arguments represent a file or directory
arguments represent a file or directory

Perhaps the runtime flag should also be injected elsewhere?

[nderjung]

Thanks @LucaSeri, I've broken up the commits to be a bit more explicit about the (re)introduction of the --runtime flag. Additionally, I've addressed the issue that surrounds not having a runtime element in a Kraftfile as this flag was not properly used in the respective runner.

[LucaSeri]

Thanks! Works well.