redesign `oci` hcl block

[dr-housemd]

Currently, oci hcl block has some confusing behaviour.

• We've reserved "pkgs" keyword to build base images.
• Users can change tag with CLI arg and also customise if they want to build a dev env or runtime env.

However, if a user wants to have different entry points for their dev env base image and runtime base image, its not possible.

Also, if a user chooses to build with go hcl block and use a base image, we dont currently support that. I think one way to solve this would be to have a parameter in oci block that can reference to other HCL attributes. This would be similar to terraform's depends_on.

[dr-housemd]

solved with https://github.com/buildsafedev/bsf/pull/112