[devcontainer] Update devcontainer scripts

[fujita-h]

Summary

• Specify the poetry version (to prevent 2.0 from being installed)
• The timing of installing packages has been unified to post start.

Checklist

[!IMPORTANT]
Please review the checklist below before submitting your pull request.

• [ ] This change requires a documentation update, included: Dify Document
• [x] I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• [ ] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [ ] I've updated the documentation accordingly.
• [ ] I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[fujita-h]

related #12368, #12369