Permit customization of the Dockerfile generated by specs (support secrets, etc.)

[geaaru]

To permit the customization of the Dockerfile generated by luet for example with:

# syntax = docker/dockerfile:experimental

we need to add special spec fields compilationspec that accepts an array of string.

Example:

compilationspec:
- "# syntax = docker/dockerfile:experimental"

[mudler]

compilationspec would clash and be a bit confusing with the current CompilationSpec struct, what about inline or inline_oci or oci_steps ?