Multi-Stage Docker Build for a Haskell (Stack) Project

Dockerfile

# The below is a combination of the following two things:
# 1) This article + corresponding Gist: https://medium.com/permutive/optimized-docker-builds-for-haskell-76a9808eb10b
# 2) This Reddit comment in response to the above:
#      - https://www.reddit.com/r/haskell/comments/cl5uod/optimized_docker_builds_for_haskell/evuzccm/
# It is a multi-stage Docker build with 3 stages: 1) dependencies, 2) build, and 3) deploy

# -------------------------------------------------------------------------------------------
# STAGE 1: Dependencies
# -------------------------------------------------------------------------------------------
FROM haskell:8.10.2 as dependencies

RUN mkdir /opt/build
WORKDIR /opt/build

# GHC dynamically links its compilation targets to lib gmp
RUN  apt-get update -y \
  && apt-get download -y libgmp10
RUN mv libgmp*.deb libgmp.deb

# First build long-term stable dependencies using YAML/etc. files you do not expect to change often
# Keep those locally in a /long-term/ sub-directory to your project's root path
COPY ./long-term/stack.yaml ./long-term/package.yaml ./long-term/stack.yaml.lock ./  
RUN stack build --dependencies-only

# If the current .yaml files in your project root have not changed from the long-term versions,
# Docker should use the cached version from the above layer. If they HAVE changed, Docker will
# rebuild the dependencies. If those changes are permanent, move the YAMLs into new /long-term/
COPY ./stack.yaml ./package.yaml ./stack.yaml.lock ./
RUN stack build --dependencies-only


# In the 'haskell' image, stack is configured with
#   - system-ghc: true
#   - install-ghc: false

# If an error is thrown by stack saying that it cannot find the correct GHC, then:
#   - You might have tried a `stack build`/etc. command in a folder lacking
#     a stack.yaml that correspnds to the version indicated in FROM above
#   - Stack then defaults to creating a ~/.stack/global-config/stack.yaml
#     from the most recent LTS resolver
#   - And will expect to find whatever compiler version that LTS points to
#   - This will happen if you built in a different WORKDIR than where you copied the YAML files

# -------------------------------------------------------------------------------------------
# STAGE 2: Build
# -------------------------------------------------------------------------------------------
FROM haskell:8.10.2 as build

# Copy stack's cache of your project's compiled dependencies
# from the previous build stage to the container
COPY --from=dependencies /root/.stack /root/.stack

# Change the container working directory to where you want to copy your project files
WORKDIR /opt/build

# Copy the entire contents of your local project folder
# to this new container (...and not just the YAMLs)
COPY . .

# Build your project
RUN stack build

# Move your compiled Haskell binary to a path easily accessible from the next stage
RUN mv "$(stack path --local-install-root --system-ghc)/bin" /opt/build/bin

# -------------------------------------------------------------------------------------------
# STAGE 3: Deploy
# -------------------------------------------------------------------------------------------
# Copy the compiled Haskell binary to a more lightweight (and less diskspace-hungry) image
# for uploading to a repository or using in deployment. 'ubuntu' is not the base image for
# the 'haskell' image, so this should probably be changed to 'debian'/etc.
FROM ubuntu:20.04 as deploy

RUN mkdir -p /opt/app
WORKDIR /opt/app

# Install libgmp using the .deb from the dependencies stage
# To-Do: Find a way to ensure the same version of libgmp is used as in the previous
#        two build stages. Have experience a downgrade at this step before. Changing
#        from Ubuntu to Debian for this final stage would probably help here.
COPY --from=dependencies /opt/build/libgmp.deb /tmp
RUN dpkg -i /tmp/libgmp.deb && rm /tmp/libgmp.deb

# Copy your compiled program from the build stage
COPY --from=build /opt/build/bin .

# Execute your app 
CMD ["/opt/app/my-dummy-app", "--verbose"]