Dev environments that match production, on the engineer’s first day.

The "set up your dev environment" doc was 14 pages and had been updated unevenly across three years. New engineers either followed it carefully (and lost a day) or skipped steps (and produced subtle bugs from missing tooling). Every PR review had a chance of finding "works on my machine" surprises.

Devcontainers per repo encoded the toolchain, the language runtime, the local-dev dependencies. Codespaces ran them in the cloud for the new-hire path — no laptop setup required. Local Docker ran them on existing laptops for the rest of the team. Backstage templates emitted the devcontainer config for new repos automatically.

New hires now ship their first PR within their first day. Works-on-my-machine tickets dropped 86%. Tooling-drift incidents (where some engineers had an older linter version) dropped to zero. The 14-page doc got archived; the README now says "open in Codespaces or run `devcontainer up`."