Dockerfile experimental syntax for properly cached cargo builds

Dockerfile

# syntax=docker/dockerfile:experimental
FROM rust:slim as builder
RUN apt-get update && \
    apt-get install musl-tools -y && \
    apt-get clean && rm -rf /var/lib/apt/lists/* && \
    rustup target add x86_64-unknown-linux-musl
COPY . .
RUN --mount=type=cache,target=/usr/local/cargo,from=rust:slim,source=/usr/local/cargo \
    --mount=type=cache,target=target \
    RUSTFLAGS=-Clinker=musl-gcc \
    cargo build \
        --target=x86_64-unknown-linux-musl \
        --release && \
    cp target/x86_64-unknown-linux-musl/release/foo /foo

FROM scratch as main
COPY --from=builder /foo /foo
ENTRYPOINT ["/foo"]