fix: remove load with multi-platform

[brivu]

Due to a limitation with docker buildx found here, users get the following error when trying to build an image with multiple platforms without pushing it:

error: docker exporter does not currently support exporting manifest lists

Currently, when a user sets the push_image parameter to false, the --load flag is automatically applied to the docker buildx command.

The --load flag is a shorthand for the combination of --output=type=docker and --push=false flags. It it instructs Docker Buildx to build the image and store it directly in the local Docker daemon without pushing it to a registry. This allows users to use the image immediately on their local machine without the need to push it to a remote registry.

However, the --load flag is not supported with multi-platform builds, resulting in the error above. To fix this issue, the --load is only set when push_image is set to false and the number of platforms is one.

[KyleTryon]

Looks good!