docker: Add rust-sdk memcheck target.

docker: Separate and tweak cargo cache paths.

ci: Improve memcheck job targets.

cargo: Add missing debug syms to bench profile.

docker: Optimize layer copying.

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

docker/Dockerfile.source

@@ -3,9 +3,6 @@
 FROM input AS source
 ARG git_checkout
 
-WORKDIR /
-COPY --link --from=input . .
-
 ADD --link --keep-git-dir . /usr/src/tuwunel
 WORKDIR /usr/src/tuwunel
 RUN <<EOF
@@ -30,7 +27,6 @@ ARG JEMALLOC_OVERRIDE
 ARG ROCKSDB_LIB_DIR
 
 WORKDIR /
-COPY --link --from=input . .
 COPY --link --from=rust ${RUST_HOME} ${RUST_HOME}
 COPY --link --from=source /usr/src/tuwunel /usr/src/tuwunel
 
@@ -67,9 +63,6 @@ ARG RUSTUP_HOME
 ARG CARGO_HOME
 ARG CARGO_TARGET
 
-WORKDIR /
-COPY --link --from=input . .
-
 WORKDIR /usr/src/tuwunel
 RUN \
 --mount=type=cache,dst=${RUSTUP_HOME}/downloads,sharing=locked \