Fix pinning Rust deps in docker images (#14129)

Author: erikjohnston

.dockerignore

@@ -9,6 +9,7 @@
 !pyproject.toml
 !poetry.lock
 !Cargo.lock
+!Cargo.toml
 !build_rust.py
 
 rust/target

changelog.d/14129.bugfix

@@ -0,0 +1 @@
+Fix an issue with Docker images causing the Rust dependencies to not be pinned correctly.

docker/Dockerfile

@@ -121,7 +121,7 @@ RUN --mount=type=cache,target=/root/.cache/pip \
 COPY synapse /synapse/synapse/
 COPY rust /synapse/rust/
 # ... and what we need to `pip install`.
-COPY pyproject.toml README.rst build_rust.py /synapse/
+COPY pyproject.toml README.rst build_rust.py Cargo.toml Cargo.lock /synapse/
 
 # Repeat of earlier build argument declaration, as this is a new build stage.
 ARG TEST_ONLY_IGNORE_POETRY_LOCKFILE