add Dagger runtime

[vito]

Adds a new Dagger runtime alongside the existing Buildkit runtime.

Note that this isn't yet feature complete, but I'd like to merge what's done so far. Dagger has proven to be a perfect for this, aside from the missing functionality. The runtime implementation is tiny!

To use the Dagger runtime, add this to ~/.config/bass/config.json:

{
  "runtimes": [
    {
      "platform": {"os": "linux"},
      "runtime": "dagger"
    }
  ]
}

All the basics are implemented and tested, but there are substantial things missing:

• Services support (container-to-container DNS)
• TLS support
• Secrets support
• Showing progress (so the UX sucks atm)
• Stable timestamps in thunk outputs
• Cache access isn't synchronized; allows multiple concurrent readers/writers
• Subpaths of caches cannot be mounted; only the toplevel
• Running a thunk from an OCI image archive

Each of these has a corresponding failing runtime test. The tests are skipped for now in CI.

The Buildkit runtime will stick around, and will remain tested with the same rigor - the shared runtime suite is very handy for that! Someday though it might make sense for the Dagger runtime to become the default, since Dagger will receive a lot of investment in out-of-the-box ease-of-use that Buildkit is unlikely to receive. We'll see how it goes.