Fix KeyError crash with Docker 29.x due to absent image config fields

[Copilot]

Docker 29.x changed docker image inspect to omit empty/nil fields (Entrypoint, Cmd, Env, User, WorkingDir) instead of returning them as null, causing gsc build to crash with a KeyError on images that lack an explicit entrypoint.

Changes

• gsc.py: Replace direct config['key'] access with config.get('key') for all five affected fields across extract_binary_info_from_image_config, extract_environment_from_image_config, and extract_user_from_image_config

# Before — KeyError on Docker 29.x when field is absent
entrypoint = config['Entrypoint'] or []
cmd = config['Cmd'] or []

# After — treats absent field same as null
entrypoint = config.get('Entrypoint') or []
cmd = config.get('Cmd') or []

• test/test_image_config.py: 20 unit tests covering both the Docker 28.x (field present, null value) and Docker 29.x (field absent) behaviors for all three affected functions

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This change is 

[mtomczew]

I can confirm this fixes issue I reported in #253

[kailun-qin]

@kailun-qin reviewed 1 file and made 1 comment.
Reviewable status: 1 of 2 files reviewed, 1 unresolved discussion, not enough approvals from maintainers (2 more required), not enough approvals from different teams (2 more required, approved so far: ) (waiting on copilot[bot] and donporter).

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a discussion (no related file):
Closing in favor of #244.