devcontainer-sync

name	devcontainer-sync
description	Keep devcontainer.json in sync when software is installed, removed, or configured inside the dev container

devcontainer-sync

Policy: Whenever you install, remove, or configure software while running inside a dev container, you MUST also update .devcontainer/devcontainer.json so the change is persisted for the next container rebuild. A change that exists only in the running container is lost on rebuild.

Detecting a dev container

We are inside a dev container when any of the following is true:

• The environment variable REMOTE_CONTAINERS is set (VS Code Dev Containers).
• The environment variable CODESPACES is set (GitHub Codespaces).
• The file /.dockerenv exists (generic Docker runtime).
• The working directory is under /workspaces/ (convention used by both VS Code and Codespaces).

Check with:

[ -n "$REMOTE_CONTAINERS" ] || [ -n "$CODESPACES" ] || [ -f /.dockerenv ] && echo "inside devcontainer"

If none of these match, we are on a native host. Skip this skill — no devcontainer update is needed.

What to update in devcontainer.json

The file is at .devcontainer/devcontainer.json. Choose the right section:

Change type	Where in devcontainer.json
apt-get install <pkg>	Add <pkg> to the apt-get install invocation inside postCreateCommand
apt-get remove <pkg>	Remove <pkg> from the apt-get install list in postCreateCommand
npm install -g <pkg>	Add/remove from the npm install -g call in postCreateCommand
pip install <pkg>	Add/remove from the pip install call in postCreateCommand
uv tool install <pkg>	Add/remove from the uv tool install calls in postCreateCommand
New feature (e.g. Docker-in-Docker)	Add an entry under "features": {}
VS Code extension	Add/remove from customizations.vscode.extensions
Environment variable	Add/remove from "remoteEnv": {}
Mounted volume	Add/remove from "mounts": []

Rules

1. Always include a comment next to the added/changed line explaining what the tool is for and which skill or CI step requires it. Match the comment style already used in the file.
2. Keep postCreateCommand as a single shell string joined with &&. Do not split it into an array — the existing format uses a string.
3. Preserve all existing entries. Only append, adjust, or remove the specific package that changed; do not reorder or reformat unrelated lines.
4. Prefer features over postCreateCommand for runtimes with an official Dev Container Feature (Node.js, Python, Go, Docker, etc.). Only fall back to postCreateCommand for packages that have no feature.
5. After editing the file, tell the user:

   "devcontainer.json updated — rebuild the container (Dev Containers: Rebuild Container) to apply the change to fresh environments."

Example — adding jq

Before (excerpt):

"postCreateCommand": "sudo apt-get update -q && sudo apt-get install -y -q lcov doxygen && npm install -g markdownlint-cli2 ..."

After:

"postCreateCommand": "sudo apt-get update -q && sudo apt-get install -y -q lcov doxygen jq && npm install -g markdownlint-cli2 ..."

Add a comment near the postCreateCommand key (or inline if the JSON allows) explaining jq is needed for the relevant script/skill.