Dockerfile: circumvent PEP-668 + fix check_connect.js path

What does this PR do?

• The latest version of Alpine Linux has rolled out a version of Python 3 adhering to PEP-668, so installing pip packages manually is not doable anymore except by using a flag to let it install anyways: pip and setuptools are now pulled from Alpine's repos through apk.
• Misskey changed path for check_connect.js: changed copy command to position it to the new correct path.

Tested on my instance + did a full build on my arm64 testbed. Confirmed working on arm64 also by user Tess/@vorap@garlic.social on the Discord server.

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