Docker + Rust + Heroku + Cargo Chef -- Small multistage build that works on Heroku with dependency caching with working SSL

Dockerfile

################################################################################
## Rust -- Recipe
################################################################################
FROM rust:latest AS chef

RUN cargo install cargo-chef

WORKDIR /var/app

FROM chef AS planner

COPY [".", "."]

RUN cargo chef prepare --recipe-path recipe.json

################################################################################
## Rust -- Build
################################################################################
FROM chef AS builder

COPY --from=planner /var/app/recipe.json recipe.json

# Create appuser
ENV USER=myapp
ENV UID=10001

RUN adduser \
    --disabled-password \
    --gecos "" \
    --home "/nonexistent" \
    --shell "/sbin/nologin" \
    --no-create-home \
    --uid "${UID}" \
    "${USER}"

# Build dependencies - this is the caching Docker layer!
RUN cargo chef cook --release --recipe-path recipe.json

# Build application
COPY [".", "."]

RUN cargo build --release --bin myapp

RUN strip -s /var/app/target/release/myapp

################################################################################
## Rust -- Application
################################################################################
FROM debian:bullseye-slim AS runtime

RUN apt-get update -y \
    && apt-get install -y --no-install-recommends openssl libssl-dev ca-certificates \
    # Clean up
    && apt-get autoremove -y \
    && apt-get clean -y \
    && rm -rf /var/lib/apt/lists/*

WORKDIR /var/app

# Import from builder.
COPY --from=builder /etc/passwd /etc/passwd
COPY --from=builder /etc/group /etc/group
COPY --from=builder /var/app/target/release/myapp /usr/local/bin
COPY ["./docker/conf", "/var/conf"]

RUN chmod +x /var/conf/run.sh

# Heroku SSH Support
# https://devcenter.heroku.com/articles/exec#using-with-docker
# COPY ["./docker/conf/heroku-exec.sh", "/app/.profile.d/"]
# RUN rm /bin/sh && ln -s /bin/bash /bin/sh

USER myapp:myapp

EXPOSE ${PORT:-80}

CMD /usr/local/bin/myapp