docker: use verbose build for ci.

Signed-off-by: Jason Volk <jason@zemos.net>

docker/Dockerfile.cargo.deps

@@ -7,6 +7,7 @@ ARG RUSTUP_HOME
 ARG CARGO_HOME
 ARG CARGO_TARGET
 ARG CARGO_TARGET_DIR
+ARG CARGO_BUILD_RUSTFLAGS
 ARG cargo_profile
 ARG cargo_features
 ARG cargo_spec_features
@@ -19,6 +20,7 @@ COPY --from=rocksdb . .
 WORKDIR /usr/src/tuwunel
 COPY --link --from=recipe recipe.json .
 
+ENV CARGO_BUILD_RUSTFLAGS=${CARGO_BUILD_RUSTFLAGS}
 RUN \
 --mount=type=cache,dst=${RUSTUP_HOME},sharing=locked \
 --mount=type=cache,dst=${CARGO_HOME},sharing=locked \