Introduce Docker staging completion handler

- [#75217152]

Signed-off-by: Peter Parente <pparent@us.ibm.com>
Log info instead of warn & error since both conditions can be induced by users

Signed-off-by: Peter Parente <pparent@us.ibm.com>
Set package_hash to image name when docker_image is set

The docker_image is the conceptual equivalent of the uploaded app bits. The package hash is normally a hexdigest of the uploaded app bits.

[#75217152]

Signed-off-by: Peter Parente <pparent@us.ibm.com>
Use more idiomatic approach for shared examples

Signed-off-by: Peter Parente <pparent@us.ibm.com>