[WIP] Card devexp_415 - Allow for custom images for builds

See the `customBuild.json` for example (will be replaced with full example, serves just for testing now ;-).

TODO:

- [ ] Add tests
- [ ] Add example
@bparees I see 'docker-builder' image requirement to be a 'default' builder image for custom build if this is not specified. I can make this required (will require validation.go update).