fix: remove forced color settings from .bashrc; set xterm variable for interactive sessions

[wambitz]

Problem

Currently, .bashrc forcibly enforces color settings via the Dockerfile (PS1, ls and grep color aliases) unconditionally. This adds color everywhere, which breaks output in CI, scripts, non-interactive processes, and non-xterm shells. It's not robust and diverges from best-practices in Docker/terminal config.

Reference: Lines 51–53 in Dockerfile

Proposal

• Remove unconditional color aliases and PS1 settings from Docker instructions.
• Instead, emit a shell block in .bashrc that checks for interactive xterm-like terminals only:

  # Color prompt and aliases only for interactive xterm-like terminals
  case "$-" in
    *i*)
      case "$TERM" in
        xterm*|screen*|tmux*)
          export IS_XTERM=1
          export PS1="\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$ "
          alias ls="ls --color=auto"
          alias grep="grep --color=auto"
          ;;
      esac
    ;;
  esac

• IS_XTERM can then be consumed by other scripts/config to enable xterm-dependent features cleanly.

Motivation

• Prevents color output from polluting logs and scripts in CI/headless/non-terminal.
• Leans on a documented, community-tested approach for terminal detection in dotfiles.
• Dockerfile remains clear, minimal, and easy to maintain.

(Optional) Further improvements

• Consider using /etc/skel/.bashrc as template for new users for consistency.
• Consider making the entire colored PS1 optional via build argument or environment variable.