Add push support

[cmodk]

Enables the push functions of buildkit.

[xtremerui]

Hi, could you give some usage examples for this PR?

oci-build-task is basically a task first of all, so adding push function is against its design pattern. Could you use a resource for pushing?

[cmodk]

Hi

Yes, sure. I am building multi platform images with IMAGE_PLATFORM and using the output i encounter the following:
#44 ERROR: docker exporter does not currently support exporting manifest lists

It seemed a little tedious to build every platform separately and then doing the manifest separately and then pushing. Even with templating, the task files grow large

[arulagrawal]

I also think building and pushing multi-arch images with concourse is too difficult as it stands (unless I'm missing something). This feature or the ability to create multi-arch images with a different format (since docker format doesn't support multi-arch) would be much appreciated.