cache elm packages of an elm project; CI can use `ELM_HOME=elm-stuff/.elm` to use the files

Dockerfile

FROM node:12.15.0-alpine3.9

RUN apk add --update make curl gzip
RUN curl -L -o elm.gz https://github.com/elm/compiler/releases/download/0.19.1/binary-for-linux-64-bit.gz && \
    gzip -d elm.gz && \
    chmod +x elm && \
    mv elm /usr/local/bin/
# following https://github.com/elm/compiler/blob/c8be24eb8e6d15f76532c3be7626c7df424c4628/installers/linux/README.md

COPY elm.json /app/elm.json
RUN mkdir /app/src && printf "module Main exposing (..)\nx = 42" > /app/src/Main.elm
RUN cd /app && ELM_HOME=/app/elm-stuff/.elm elm make src/Main.elm --output /dev/null
# elm packages now cached in `/app/elm-stuff/.elm`

Makefile

# What to do when Elm Package Registry is down?
# https://discourse.elm-lang.org/t/what-to-do-when-elm-package-registry-is-down/5048/6
#
# TLDR:
# 	cache the $ELM_HOME directory such that you do not need to talk to
# 	package.elm-lang.org or to github.com unless a new package dependency was added.
#
# This target `elm-stuff/.elm` should only re-run when `elm.json` is more recent
#
# 1. Running `make elm-stuff/.elm` will
#     a. spin up Docker and cache the packages used in `elm.json`; all steps are cached
#     b. always export the cached packages to local disk on `elm-stuff/.elm`
#
# 2. From this point onwards, any CI process that need cached packages can
#    simply set `ELM_HOME=elm-stuff/.elm`
# 
# NOTE: path `elm-stuff/.elm` was picked so there's no extra subdirectory to gitignore
elm-stuff/.elm: elm.json
	tar -cf - Dockerfile elm.json | docker build - -f Dockerfile --iidfile iidfile
	docker run --detach --cidfile cidfile `cat iidfile`
	mkdir -p elm-stuff
	docker cp `cat cidfile`:/app/elm-stuff/.elm elm-stuff/
	docker rm -f `cat cidfile`
	rm -f cidfile iidfile
	touch elm-stuff/.elm