Update Dockerfile to build in stages for caching

Dockerfile

@@ -1,14 +1,28 @@
-FROM rust:1.62
-
-WORKDIR /usr/src/oxen-server
+FROM rust:1.62 as builder
 
 USER root
 RUN apt-get update && apt-get install --assume-yes apt-utils
 RUN apt-get install -y libclang-dev
+RUN cargo install cargo-build-deps
+
+# create an empty project to install dependencies
+RUN cd /usr/src && cargo new --bin oxen-server
+WORKDIR /usr/src/oxen-server
+COPY Cargo.toml Cargo.lock ./
+COPY src/lib/Cargo.toml src/lib/Cargo.toml
+COPY src/cli/Cargo.toml src/cli/Cargo.toml
+COPY src/server/Cargo.toml src/server/Cargo.toml
+# build just the deps for caching
+RUN cargo build-deps --release
 
-COPY . .
-RUN cargo install --path .
+# copy the rest of the source and build the server
+COPY src src
+RUN cargo build --release --bin oxen-server
 
+# Minimal image to run the binary (without Rust toolchain)
+FROM debian:bullseye-slim AS runtime
+WORKDIR /oxen-server
+COPY --from=builder /usr/src/oxen-server/target/release/oxen-server /usr/local/bin
 ENV SYNC_DIR=/var/oxen/data
 EXPOSE 3000
-CMD ["oxen-server", "start"]
+CMD ["oxen-server", "start"]