Replace image-build.sh with new functionality from code engine module

[ocofaigh]

In terraform-ibm-modules/terraform-ibm-code-engine#210 the code engine module will have the ability to build - so we should use it instead of the custom image-build.sh script we currently have in https://github.com/terraform-ibm-modules/terraform-ibm-watsonx-self-managed-ocp/blob/main/modules/cpd-image-build/main.tf