just coverage could not run inside the container because bin/coverage
needs both tools and neither shipped in the image.

Assisted-by: GLM-5.3 via pi

diff --git a/.ramekin/Dockerfile b/.ramekin/Dockerfile
index b8b0409..3213fea 100644
--- a/.ramekin/Dockerfile
+++ b/.ramekin/Dockerfile
@@ -7,3 +7,10 @@ RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs \
     && /root/.cargo/bin/rustup component add clippy rustfmt llvm-tools
 
 ENV PATH="/root/.cargo/bin:$PATH"
+
+# bin/coverage needs grcov for the report and bc for the threshold
+# comparison. grcov is pinned so report behavior is stable.
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends bc \
+    && rm -rf /var/lib/apt/lists/* \
+    && cargo install grcov --version 0.10.7 --locked