why cacheImage requires --publish?

[lookis]

can I push image manually? just reuse the image from registry

[natalieparellano]

@lookis a bit late here, but you may find the recently added --cache flag helpful:

      --cache cache                 Cache options used to define cache techniques for build process.
                                    - Cache as bind: type=<build/launch>;format=bind;source=<path to directory>;
                                    - Cache as image: type=<build/launch>;format=image;name=<registry image name>;
                                    - Cache as volume: type=<build/launch>;format=volume;[name=<volume name>;]
                                        - If no name is provided, a random name will be generated.

This would allow you to export the application image to a docker daemon while caching to a registry. The cache image should be in a registry for performance reasons (data retrieval from the daemon can be quite slow).