docker(launcher): include Node.js runtime

[yzxlr]

Summary\n- switch the goreleaser launcher image from plain Alpine to the same Node-based runtime family used by the full image\n- document that the launcher container now includes Node.js for npm-based MCP servers and scripts\n- add a regression test that asserts the launcher Dockerfile keeps using a Node runtime base image\n\n## Testing\n- go test ./docker\n\n## Notes\n- I could not run docker build locally because Docker is not installed in this environment.\n\nCloses #1371

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