Bundle with docker-compose installs gems from scratch - ruby-on-rails

Question

I'm trying to cache bundle install on Docker build.
I've tried recommendations from several blog posts and I ended up in something like this:
Dockerfile:
FROM ruby:2.5.0-alpine
RUN apk update && apk add build-base postgresql-dev nodejs git tzdata
ENV APP_HOME /panabus-api
RUN mkdir $APP_HOME
WORKDIR $APP_HOME
COPY Gemfile Gemfile.lock ./
ENV BUNDLE_PATH /gems
ENV GEM_PATH /gems
ENV GEM_HOME /gems
RUN bundle check || bundle install
COPY . $APP_HOME
ENV RAILS_ENV=production
RUN bundle exec rake RAILS_ENV=production assets:precompile apipie:cache
LABEL maintainer="Hari Carreras Pérez <hc#abtion.com>"
CMD bundle exec puma -C config/puma.rb
docker-compose.yml
version: '3'
services:
box:
image: busybox
volumes:
- gem_cache:/gems
postgres:
image: postgres:10.3-alpine
ports:
- 5432:5432
volumes:
- postgres:/var/lib/postgresql/data
restart: always
web:
build: .
image: panabus-api-web
command: bundle exec puma -C config/puma.rb -p 3000
depends_on:
- postgres
- box
volumes:
- .:/panabus-api
- gem_cache:/gems
ports:
- "3000:3000"
restart: always
environment:
- RAILS_ENV=development
tty: true
stdin_open: true
setup:
build: .
depends_on:
- postgres
- box
environment:
- RAILS_ENV=development
volumes:
- .:/panabus-api
- gem_cache:/gems
command: "bin/rails db:create db:migrate"
volumes:
postgres:
gem_cache:
This doesn't update the gems if the Gemfile hasn't changed. Unfortunately, if it has changed, it install all gems from scratch, not using the cache.
Is there any way to cache the gems on build?
I'd like to keep bundle in the Dockerfile since Heroku uses the same Dockerfile.

Answer 1

You need to add a tag to your bundle check where you have cached the installed gems
bundle check || bundle install --binstubs="$BUNDLE_BIN"
and cache the bundle with a specific path like so
ENV BUNDLE_PATH=/bundle \
BUNDLE_BIN=/bundle/bin \
GEM_HOME=/bundle
ENV PATH="${BUNDLE_BIN}:${PATH}"