Default to BASH in reference .devcontainer Docker image

This ticket is a request to simplify the reference implementation for a Claude Code .devcontainer by removing ZSH and its customizations from the image and instead using BASH. Rationale:

1. The best practices mention BASH, implying that Claude Code behaves better in BASH environments: https://www.anthropic.com/engineering/claude-code-best-practices
2. Adding ZSH and personalizations like the "powerline10k" theme create bloat in the Docker image and include one person's personal preferences which don't match standard defaults that most users would expect.

Consider instead making this reference Docker image focus only on essential packages needed for an effective Claude Code environment and exclude customizations based on personal preference.

Thanks!