Attempt at doing fast cached rebuilds with stack.

Dockerfile

# syntax = docker/dockerfile:experimental
# Build with DOCKER_BUILDKIT=1 while support for caching features is not 'on by default' in Docker.

FROM ubuntu:18.04

ENV STACK_VERSION=1.9.3 STACK_SHA256SUM=c9bf6d371b51de74f4bfd5b50965966ac57f75b0544aebb59ade22195d0b7543

RUN wget --no-verbose https://github.com/commercialhaskell/stack/releases/download/v${STACK_VERSION}/stack-${STACK_VERSION}-linux-x86_64-static.tar.gz \
 && echo ${STACK_SHA256SUM} stack-${STACK_VERSION}-linux-x86_64-static.tar.gz | sha256sum --check --strict \
 && tar \
        --directory /usr/local/bin \
        --file stack-${STACK_VERSION}-linux-x86_64-static.tar.gz \
        --strip-components=1 \
        --extract stack-${STACK_VERSION}-linux-x86_64-static/stack

WORKDIR /app

# Copy in the stack configuration and run stack update.
COPY stack.yaml .

RUN --mount=type=cache,target=/root/.stack,id=dotstack \
    --mount=type=cache,target=/app/.stack-work,id=stack-work \
    \
    stack update

# Copy in the cabal files, so that we can install dependencies. This is done as
# a separate step so that dependencies don't need to be invalidated for typical
# builds - only if these files are changed.
COPY project.cabal .
COPY subproject/subproject.cabal subproject/subproject.cabal
COPY subproject1/subproject1.cabal subproject1/subproject1.cabal
COPY subproject2/subproject2.cabal subproject2/subproject2.cabal
COPY subproject3/subproject3.cabal subproject3/subproject3.cabal
COPY subproject4/subproject4.cabal subproject4/subproject4.cabal

# Fetch dependencies from the network.
RUN --mount=type=cache,target=/root/.stack,id=dotstack \
    --mount=type=cache,target=/app/.stack-work,id=stack-work \
    \
    stack build --prefetch --dry-run --test --bench --flag some-flags-are-passed

# Build dependencies.
RUN --mount=type=cache,target=/root/.stack,id=dotstack \
    --mount=type=cache,target=/app/.stack-work,id=stack-work \
    \
    stack build --dependencies-only --test --bench --flag some-flags-are-passed

COPY . /app

# Build the project and subprojects.
RUN --mount=type=cache,target=/root/.stack,id=dotstack \
    --mount=type=cache,target=/app/.stack-work,id=stack-work \
    \
    stat subproject1/subproject1.cabal \
 && stack build --test --bench --no-run-benchmarks --no-run-tests --flag some-flags-are-passed