Fix available images without image_name

[MukundaKatta]

Description

• flatten Docker images into tag strings for GET /docker/available-images/ when image_name is omitted
• keep the existing filter behavior when image_name is provided
• add dependency-light unit coverage for the tag flattening and filtering behavior

Fixes #160

Testing

• python3 -m pytest test/test_docker_router.py test/test_portfolio_state.py
• git diff --check

Note: test/test_portfolio_state.py is skipped in this checkout because the optional hummingbot package is not installed.