scuba: Default _get_image_config() to return None if Config key is missing

Docker Engine 28.2 changed the output of 'docker inspect --type image'
to no longer include keys when their values are 'null'. This could cause
get_image_command() and get_image_entrypoint() to raise an exception if
the Cmd or Entrypoint keys are missing, respectively.

Change from [] to .get() to return None if the key is missing, restoring
the previous behavior.

Fixes #262

Author: JonathonReinhart

CHANGELOG.md

@@ -3,6 +3,9 @@ All notable changes to this project will be documented in this file.
 This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
+### Fixed
+- Fixed regression with Docker Enginer 28.2 (#269)
+
 ### Removed
 - Dropped support for Python 3.7 (#242)
 

scuba/dockerutil.py

@@ -113,7 +113,7 @@ def get_images() -> Sequence[str]:
 def _get_image_config(image: str, key: str) -> Optional[Sequence[str]]:
     info = docker_inspect_or_pull(image)
     try:
-        result = info["Config"][key]
+        result = info["Config"].get(key)
     except KeyError as ke:
         raise DockerError(f"Failed to inspect image: JSON result missing key {ke}")