fix(docker): fall back to image's default userDir via NODE_RED_CLI_DEFAULT_USERDIR

[tbrandenburg]

Issues fixed

Closes #31

Summary

--docker previously never pointed Node-RED's userDir at a community image's own pre-populated default userDir (e.g. /data in agentic-workflow-dev-env), forcing redundant --node-modules re-installs even when a package was already baked into the image.

This adds an opt-in env-var convention, resolved inside the container by the existing shared runFlowInvocation() in src/run-envelope.js:

• New env var: NODE_RED_CLI_DEFAULT_USERDIR. Image authors set this to their own conventional userDir path.
• Precedence:
  1. Explicit --user-dir always wins (unchanged).
  2. Else, if NODE_RED_CLI_DEFAULT_USERDIR is set and points at an existing directory: used as userDir, never deleted afterward (image-owned).
  3. Else: falls back to today's ephemeral tmpdir — zero behavior change for every image that doesn't opt in.
  4. If the env var is set but invalid (missing/not-a-directory/inaccessible): fails open — logs a one-line stderr warning and falls back to the ephemeral tmpdir, rather than aborting.
• No new CLI flag needed.

Files changed

• src/run-envelope.js: resolveDefaultUserDir() helper + precedence wiring in runFlowInvocation.
• bin/node-red-cli-sandbox-entry.js: doc-comment only.
• bin/node-red-cli.js: one-line HELP_TEXT note for image authors.
• README.md: documents the new convention under --docker.
• test/unit/run-envelope.unit.test.js (new): unit coverage for resolveDefaultUserDir (unset / valid dir / nonexistent path / file-not-dir).
• test/integration/run-envelope.integration.test.js: env-var userDir used + never removed; explicit --user-dir precedence.
• test/integration/docker.integration.test.js: real-Docker regression test reproducing the issue's exact scenario (pre-baked module discovered via --docker alone) + precedence test (--user-dir overrides the env var).

Validation

• make lint — pass
• make format — pass
• make test — 130/130 passed, 0 skipped (Docker was available in the CI/dev environment, so all new Docker integration tests actually ran against real containers, not skipped)
• make audit — 0 vulnerabilities (ran as part of make ci via the pre-push hook)

E2E coverage

No e2e changes needed — this is a Docker/env-var wiring concern already covered by the new Docker integration tests, per the issue's own agreed testing strategy (integration-level, real Docker + real embedded Node-RED, is the right layer; full test/e2e/ example-flow run adds no additional signal here).

Risks / follow-ups

• Low risk: fully opt-in (image must set the env var), fails open on misconfiguration, and every existing test path (which doesn't set the var) is unchanged.
• Follow-up (out of scope for this repo): the reference image ghcr.io/tbrandenburg/agentic-workflow-dev-env would need its own change to export NODE_RED_CLI_DEFAULT_USERDIR=/data to benefit from this.

Unrelated issues found

• None.