Docs - intro-invocation-images.md update

[phillipgibson]

          ```suggestion

Porter starts the Dockerfile by using a base image. You can customize the base image by specifying a Dockerfile template in the porter.yaml. By default, Porter only targets a single os/architecture(linux/amd64) for the bundle installer. If you want to use other platform, feel free to change the platform flag in the generated Dockerfile template. Next, a set of CA certificates is added. Then, contents of the current directory are copied into the bundle directory (/cnab/app) in the bundle installer. This will include any contributions from the mixin executables. Finally, an entry point that conforms to the CNAB specification is added to the image.

_Originally posted by @schristoff in https://github.com/getporter/porter/pull/2790#discussion_r1243116007_