Draft: Swift version management, Docker multi-stage build

[josefdolezal]

While implementing #38, I sometime struggled with steps necessary to make the app build. I'd like to pitch developer experience improvement using swiftenv for toolchain management.

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The problem

The current toolchain management is hard to understand and error-prone when managing multiple versions. The configuration of supported swift versions is spread across multiple places (build scripts, Vapor app, frontend and Docker) making maintenance real challenge.

The solution

• Offload swift toolchain management to swiftenv
• Use installed toolchains (swiftenv versions) as the source of truth for BE and FE (using new /api/versions endpoint)
• Drop per-toolchain OnlinePlayground in favour of Swift 5 ABI (build once, link everywhere™️)
• (Unrelated) Use slim container for app deployment
  • Uses Docker multi-stage build (stripping final image from system swift version, node, ..)

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Let me know if it sounds interesting to you and we can coordinate to finish this PR.