Set environment variables and mounts in devcontainer.json: containerEnv vs remoteEnv, ${localEnv}/${containerEnv} substitution, and named-volume cache mounts

Documented steps

1. Use containerEnv for variables that all processes in the container should see: {"containerEnv": {"NODE_ENV": "development"}} — set at container creation, changing it requires recreating the container
2. Use remoteEnv for variables only the connecting tool and its subprocesses (terminals, tasks, debugging) should see: {"remoteEnv": {"PATH": "${containerEnv:PATH}:/custom/bin"}}
3. Pull values from the host with ${localEnv:VAR_NAME} (optionally ${localEnv:VAR_NAME:default} for a fallback)
4. Reference variables already set in the container with ${containerEnv:VAR} — supported in remoteEnv (e.g. to extend PATH)
5. Add persistent caches with mounts: {"mounts": [{"source": "npm-cache", "target": "/home/vscode/.npm", "type": "volume"}]} — named volumes survive container rebuilds
6. Bind-mount extra host folders with {"source": "${localWorkspaceFolder}/../shared", "target": "/shared", "type": "bind"}
7. Docs: https://containers.dev/implementors/json_reference/