docker: play nicely with cargo's incremental compilation

.dockerignore

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

Dockerfile

@@ -4,13 +4,19 @@ RUN apt-get update \
     && DEBIAN_FRONTEND=noninteractive \
     apt-get install --no-install-recommends --assume-yes \
     protobuf-compiler libprotobuf-dev
-RUN echo "fn main() {}" > dummy.rs
-COPY mpc-recovery/Cargo.toml Cargo.toml
-RUN sed -i 's#src/main.rs#dummy.rs#' Cargo.toml
-RUN sed -i 's#mpc-recovery-gcp = { path = "../mpc-recovery-gcp" }##' Cargo.toml
-RUN cargo build --release
 COPY . .
-RUN cargo build --release --package mpc-recovery
+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
 
 FROM debian:buster-slim as runtime
 RUN apt-get update && apt-get install --assume-yes libssl-dev