fix(initrd): Correctly propagate workdir to dockerfile

[LucaSeri]

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

Commit 5cb57ad reworked the way the workdir is passed to different initrd implementation. This ended up
in having a duplicate workdir entry in the dockerfile initrd type. Changes take into account that with the new implementation, the path of the dockerfile is relative to the workdir.

This change ultimately solves an issue related to kraftkit not being able to find the dockerfile when building outside the cwd:

$ kraft build ../scss/redis/
<!> building rootfs                                                                                                                                                                                                                                               x86_64 [2.8s]
 i  creating ephemeral buildkit container
 i  #1 [internal] load build definition from Dockerfile
 i  #1 transferring dockerfile: 2B done
 i  #1 DONE 0.0s
 E  could not wait for err group: could not solve: failed to solve: failed to read dockerfile: open Dockerfile: no such file or directory