feat(init): warn when a Python project has no Dockerfile

[patrikbraborec]

apify init doesn't create a Dockerfile. When a Python project has none, the platform builds it from the default apify/actor-node:24 image. The build succeeds, and the Actor then fails at runtime in a confusing way. This came up during Devin plugin testing.

• apify init now warns when it detects a Python project with no Dockerfile. It checks the --dockerfile flag, the actor.json dockerfile field, .actor/Dockerfile and a root Dockerfile, which is the platform's lookup order.
• Two local tests: the warning appears when no Dockerfile exists, and it doesn't when one does.
• No flag, argument or description changes, so no update-docs. No dependency changes.

Related: #1479 and #1330 fix the other init problems from the same testing (--yes not skipping the name prompt, and the hang without a TTY).

[patrikbraborec]

Closing: a warning that only covers Python packages misses most cases (Python with a package.json, other languages, loose scripts that init rejects). The missing-Dockerfile problem needs a broader fix. For now the skills tell agents to create a Dockerfile (apify/agent-skills#98).