kraft fails to build a Unikernel if relative path is specified

[ajpetersons]

Describe the bug

When specifying a directory to build with kraft build [DIR] that is not the current working directory, the build process fails when assessing initramfs:

E  getting initramfs size: stat ./Dockerfile: no such file or directory

Steps to reproduce

1. Clone catalog repository (tested at https://github.com/unikraft/catalog/tree/5a22d73139c02021e863f637383e1dc94d965564)
2. Attempt to build nginx: kraft build library/nginx/1.25 -p fc

Expected behavior

Nginx image should build successfully

KraftKit version

Build fails when getting initramfs size:

 E  getting initramfs size: stat ./Dockerfile: no such file or directory

Which architectures were you using or does this bug affect?

No response

Which operating system were you using or does this bug affect?

No response

Relevant log output