Add example of building a Dockerfile with local tar file build context

[njlr]

This PR adds an example of using a Dockerfile with a local tar as a build context.

This is not the ideal approach, which this is clearly signposted in the README!, but it shows an easy migration path from rules_docker. The existing example does not show how Bazel build artefacts can be passed to BuildX.

How it works:

1. Create a tar file using rules_pkg
2. Pipe the tar file into BuildX using sh_binary and run_binary
3. Use the BuildX output as a base for an oci_image

This is a replacement for container_run_and_commit.

The example could easily be adapted to behave like container_run_and_extract by changing the --output flag to a tar.