fix: image scan failing on multi architecture image

[kyujin-cho]

This PR adds support for handling multi-architecture image manifest. This PR closes lablup/backend.ai#373.

[codecov]

Codecov Report

Merging #543 (aa0b08b) into main (3d5fda0) will increase coverage by 0.36%.
The diff coverage is 42.32%.

@@            Coverage Diff             @@
##             main     #543      +/-   ##
==========================================
+ Coverage   48.82%   49.18%   +0.36%     
==========================================
  Files          54       54              
  Lines        8951     9033      +82     
==========================================
+ Hits         4370     4443      +73     
- Misses       4581     4590       +9     

Impacted Files	Coverage Δ
src/ai/backend/manager/api/events.py	35.90% <0.00%> (-0.14%)	⬇️
src/ai/backend/manager/api/utils.py	58.08% <0.00%> (-0.22%)	⬇️
src/ai/backend/manager/models/domain.py	58.28% <0.00%> (ø)
src/ai/backend/manager/models/group.py	45.51% <0.00%> (ø)
src/ai/backend/manager/models/session_template.py	31.64% <0.00%> (ø)
src/ai/backend/manager/scheduler/dispatcher.py	25.40% <0.00%> (-0.14%)	⬇️
src/ai/backend/manager/registry.py	16.97% <6.25%> (-0.14%)	⬇️
src/ai/backend/manager/idle.py	30.38% <33.33%> (-0.47%)	⬇️
src/ai/backend/manager/models/agent.py	51.76% <33.33%> (-0.34%)	⬇️
src/ai/backend/manager/server.py	61.57% <37.77%> (+1.47%)	⬆️
... and 17 more

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