Use a cache mount to speed up rebuilding compute node image (#10737)

Building the compute rust binaries from scratch is pretty slow, it takes
between 4-15 minutes on my laptop, depending on which compiler flags and
other tricks I use. A cache mount allows caching the dependencies and
incremental builds, which speeds up rebuilding significantly when you
only makes a small change in a source file.

compute/compute-node.Dockerfile

@@ -1578,7 +1578,15 @@ ENV BUILD_TAG=$BUILD_TAG
 USER nonroot
 # Copy entire project to get Cargo.* files with proper dependencies for the whole project
 COPY --chown=nonroot . .
-RUN mold -run cargo build --locked --profile release-line-debug-size-lto --bin compute_ctl --bin fast_import --bin local_proxy
+RUN --mount=type=cache,uid=1000,target=/home/nonroot/.cargo/registry \
+    --mount=type=cache,uid=1000,target=/home/nonroot/.cargo/git \
+    --mount=type=cache,uid=1000,target=/home/nonroot/target \
+    mold -run cargo build --locked --profile release-line-debug-size-lto --bin compute_ctl --bin fast_import --bin local_proxy && \
+    mkdir target-bin && \
+    cp target/release-line-debug-size-lto/compute_ctl \
+       target/release-line-debug-size-lto/fast_import \
+       target/release-line-debug-size-lto/local_proxy \
+       target-bin
 
 #########################################################################################
 #
@@ -1781,15 +1789,15 @@ RUN mkdir /var/db && useradd -m -d /var/db/postgres postgres && \
 COPY --from=awscli /usr/local/aws-cli /usr/local/aws-cli
 
 COPY --from=postgres-cleanup-layer --chown=postgres /usr/local/pgsql /usr/local
-COPY --from=compute-tools --chown=postgres /home/nonroot/target/release-line-debug-size-lto/compute_ctl /usr/local/bin/compute_ctl
-COPY --from=compute-tools --chown=postgres /home/nonroot/target/release-line-debug-size-lto/fast_import /usr/local/bin/fast_import
+COPY --from=compute-tools --chown=postgres /home/nonroot/target-bin/compute_ctl /usr/local/bin/compute_ctl
+COPY --from=compute-tools --chown=postgres /home/nonroot/target-bin/fast_import /usr/local/bin/fast_import
 
 # pgbouncer and its config
 COPY --from=pgbouncer         /usr/local/pgbouncer/bin/pgbouncer /usr/local/bin/pgbouncer
 COPY --chmod=0666 --chown=postgres compute/etc/pgbouncer.ini /etc/pgbouncer.ini
 
 # local_proxy and its config
-COPY --from=compute-tools --chown=postgres /home/nonroot/target/release-line-debug-size-lto/local_proxy /usr/local/bin/local_proxy
+COPY --from=compute-tools --chown=postgres /home/nonroot/target-bin/local_proxy /usr/local/bin/local_proxy
 RUN mkdir -p /etc/local_proxy && chown postgres:postgres /etc/local_proxy
 
 # Metrics exporter binaries and configuration files