fix missing manifest when performing multi-platform build and push

[jshbrntt]

Fixes #1062.

[chrmarti]

Thanks for the PR! I think going from --load to automatic --push as a side-effect of adding another platform might be unexpected to users.

[jshbrntt]

@chrmarti how else would you suggest we fix the linked issue?

Doesn't --load and --push effectively result in the same operation?

I think the images built will already automatically loaded into the local dockerd so --load is kind of redundant?