FEATURE: Allow Docker images to be built remotely instead of on the host

_This issue was originally opened by @dnk8n as hashicorp/packer#7963. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

Why? For the same reason ebs-builder doesn't install everything locally and leave you with an AMI on your own dev machine, then push to Amazon.

I speak specifically about Amazon AWS, but other cloud providers should also apply (so this is a general request which applies to all builders)

Use-case: The host is often a docker container with minimal resources, using docker image: `hashicorp/packer:1.4.2` for example. In many cases the `docker run` command is locked down as well.

It would be great if you could allow a similar workflow to ebs-builder, except end up with a Docker image being pushed instead of an AMI. That is, instead of provisioning locally, a temporary instance is created in ones' own VPC to do the heavy lifting.

Currently, everything is in place to allow that (if Packer and Docker are pre-installed on the remote instance which does the provisioning), except you are forced to create an AMI you don't care about.

Another use cases, say I need 8GB of RAM to compile something, then it would be useful to be able to use Packer's functionalities to use a tiny 'pilot' CI/CD runner to just run packer, with all heavy lifting done in one's own VPC.