Technical: Install and configure Zsh/OhMyZsh

[mtthwcmpbll]

Context:
The default shell is likely sh or bash with a minimal prompt. For a "turtorial" aimed at modern devs, a richer shell experience is preferred.

Implementation Plan:

1. Dockerfile:
   • Install zsh.
   • Install oh-my-zsh (via script).
   • Install spaceship-prompt (or a lightweight alternative if spaceship is too heavy).
   • Configure .zshrc for the turtorial user.
   • Set zsh as the default command/entrypoint.

Acceptance Criteria:

• Container builds successfully.
• Terminal session starts with zsh.
• Prompt is styled (colors, git status, etc.).
• Common aliases (e.g., ll) work.