fix(docker): stop excluding Cargo.lock from the build context

The .dockerignore listed Cargo.lock, which excluded it from the Docker build
context and broke 'COPY Cargo.toml Cargo.lock' in the Dockerfile (the new
multi-stage build surfaces this — the old images were built before the COPY
existed). Cargo.lock is committed for this binary crate and must be available
to the image build for reproducible dependency resolution.

Verified: build now reaches the dependency-caching step.

backend/.dockerignore

@@ -1,2 +1,3 @@
 target/
-Cargo.lock
+# Note: do NOT exclude Cargo.lock here. It is committed (this is a binary crate)
+# and the Dockerfile must COPY it for reproducible dependency resolution.