Let Github runners reuse caches for Rust and Nixpkgs

osdk/tools/docker/Dockerfile

@@ -145,6 +145,12 @@ RUN curl https://sh.rustup.rs -sSf | \
     && cargo -V \
     && rustup component add rust-src rustc-dev llvm-tools-preview
 
+# Let Github runners reuse the Rust toolchain installed in the Dockerfile and
+# also the Cargo crates downloaded subsequently in the Docker build process.
+RUN mkdir -p /github/home \
+    && ln -s /root/.cargo /github/home/.cargo \
+    && ln -s /root/.rustup /github/home/.rustup
+
 # Install cargo tools
 RUN cargo install \
     cargo-binutils \

tools/docker/nix/Dockerfile

@@ -39,6 +39,10 @@ RUN make general_pkgs VERBOSE=0 \
 # Clean source files
 RUN rm -rf /root/test
 
+# Let Github runners reuse Nixpkgs tarball installed in the Dockerfile.
+RUN mkdir -p /github/home/.cache \
+    && ln -s /root/.cache/nix /github/home/.cache/nix
+
 VOLUME [ "/root/asterinas" ]
 
 WORKDIR /root/asterinas