[bug#67613] Introduce unit tests for oci-container-service-type.

[paul]

Hi Ludo’ ,

I should have created a suitable system test for the 
oci-container-service-type. Thanks to a nice input from 
@graywolf@emacs.ch on mastodon, and actually to be able to run the test 
since the vm doesn't have internet access and can't pull OCI images, I 
implemented a new oci-image record that can be given some lowerable 
value that can be lowered to an OCI tarballed image and passed to the 
image field of the oci-container-configuration record. I'd like to point 
out two things:

- It's the first time I use Guix internal API to build derivations, I 
took most of my implementation from other places around Guix and I hope 
is sound but I may have missed something. I'd like your feedback about it.

- I was tempted to make the image field of the 
oci-container-configuration record directly only accept oci-image 
records (hence making the value field of oci-image optional) but that 
would break existing configurations. I'm not sure about the contract we 
have for configuration records API, should I wait 1.5.0 for this change?


I'm sending an updated patchset, thank you for all your help and efforts.


giacomo