docker: util RUN --mount=type=cache...

downside is this doesn't play well with

BuildKit's cache-to, cache-from. affecting our ability to push cache to GH

moby/buildkit#1673

.dockerignore

@@ -4,6 +4,5 @@
 .gitattributes
 
 target/
-!target/cache
 Dockerfile
 .dockerignore

Dockerfile

@@ -5,22 +5,17 @@ RUN apt-get update \
     apt-get install --no-install-recommends --assume-yes \
     protobuf-compiler libprotobuf-dev
 COPY . .
-COPY ./targe[t]/cach[e]/us[r]/ /usr/
-RUN rm -rf ./target/cache
-RUN CARGO_INCREMENTAL=0 cargo build --release --package mpc-recovery
-
-FROM scratch as export-artifacts
-COPY --from=builder /usr/src/app/target /usr/src/app/target
-COPY --from=builder /usr/local/cargo/bin /usr/local/cargo/bin
-COPY --from=builder /usr/local/cargo/git* /usr/local/cargo/git
-COPY --from=builder /usr/local/cargo/.crate.toml* /usr/local/cargo/.crate.toml
-COPY --from=builder /usr/local/cargo/.crate2.toml* /usr/local/cargo/.crate2.toml
-COPY --from=builder /usr/local/cargo/registry/cache /usr/local/cargo/registry/cache
-COPY --from=builder /usr/local/cargo/registry/index /usr/local/cargo/registry/index
+RUN --mount=type=cache,target=/usr/src/app/target \
+    --mount=type=cache,target=/usr/local/cargo/git \
+    --mount=type=cache,target=/usr/local/cargo/registry/cache \
+    --mount=type=cache,target=/usr/local/cargo/registry/index \
+    CARGO_INCREMENTAL=0 cargo build --release --package mpc-recovery \
+    && \
+    cp ./target/release/mpc-recovery /usr/local/bin/mpc-recovery
 
 FROM debian:buster-slim as runtime
 RUN apt-get update && apt-get install --assume-yes libssl-dev
-COPY --from=builder /usr/src/app/target/release/mpc-recovery /usr/local/bin/mpc-recovery
+COPY --from=builder /usr/local/bin/mpc-recovery /usr/local/bin/mpc-recovery
 WORKDIR /usr/local/bin
 
 ENTRYPOINT [ "mpc-recovery" ]