Dockerfile: copy the shared backend crate into the image build

[eetu]

Create and publish Docker images has been failing on main since #217, with:

error: failed to load manifest for workspace member `/app/apps/tracker/backend`
Caused by: failed to load manifest for dependency `scene-backend`
Caused by: failed to read `/app/crates/scene-backend/Cargo.toml`

Not caused by #217 — the Dockerfile has been wrong since crates/scene-backend landed in #213, and the workspace-deps layer was simply cached until something invalidated it. Two omissions:

1. The cache-warm stage copied four crate manifests but not crates/scene-backend/Cargo.toml. Cargo parses the whole workspace before building any of it, so a member whose manifest it cannot read is fatal — regardless of which packages -p names.
2. No stage copied that crate's real src. So even with the manifest present, the three backends would have compiled against the empty stub lib and failed on every item they use.

The manifest and a stub lib join the warm stage; the real source lands in one shared-build stage that all three backend stages now extend, so a change to the shared crate rebuilds the three of them off the same layer.

Verified two ways. Locally without a container: reproducing the stage's exact copy set in a temp dir reproduces that identical cargo error, and adding the manifest makes cargo metadata parse. Then for real — this workflow is push: [main] only, so I dispatched it on this branch: all three images (tracker, party, transcoder) built and pushed green. That run left throwaway :docker-shared-crate tags in GHCR; I'll delete them once this merges.