Add --image flag to run a custom emulator image for one start

[anisaoshafi]

What

Adds an --image flag, so you can run any image without editing config.toml:
lstk -t snowflake --image localstack/snowflake-next

How it works

• --image applies to one run only. lstk never saves it to config.
• -t works as before.
• stop, status, and logs still find the emulator. lstk finds its container by name, not by image.
• restart keeps the image. lstk puts a label on a container started with --image, and a plain restart reuses that image. If the image came from config, restart reads the config again, so config edits still apply.
• lstk rejects invalid image names, and a known image for the wrong emulator (e.g. -t aws --image localstack/snowflake). Neither changes the config.
• If the emulator is already running another image, lstk warns that --image was not applied.

Testing

• New integration tests cover the flag, the checks, and restart.
• Ran it by hand with the real localstack/snowflake-next image: start, status, restart, stop, and the interactive mode.

scenario	result
start success
status
known image of the wrong type
unknown image