chore: add .dockerignore to optimize Docker build

[xu826Jamin]

This PR fixes #69

Description

• Added a .dockerignore file to prevent unnecessary files from being
  copied into the Docker build context.
• Excludes Python virtual environments (venv/, __pycache__, *.pyc)
• Excludes Node dependencies (node_modules/)
• Excludes Git metadata (.git/)
• Excludes environment files (.env)
• Excludes OS and IDE specific files (.DS_Store, .vscode/)

Additional context

• Without this file, Docker copies the entire project including dependencies
  and cache files, making builds unnecessarily slow and images larger.
• This is a zero-risk change that only affects Docker build performance.

[Copilot]

Pull request overview

Adds a new repo-root .dockerignore intended to reduce Docker build context size by excluding VCS metadata, dependency directories, caches, and local environment/editor files.

Changes:

• Introduces a .dockerignore with ignore rules for Git, Python, Node, build outputs, env files, and IDE/OS artifacts.

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[Copilot]

With the current docker-compose.yml, both services build from subdirectory contexts (./gdbui_server and ./webapp) and specify DockerFile.dev in those directories. Docker only applies .dockerignore from the build context root, so a repo-root .dockerignore will not affect these builds. To actually optimize builds, add .dockerignore files inside gdbui_server/ (backend) and/or update the compose build contexts to the repo root (and adjust dockerfile paths accordingly).