Add docker instructions for development

CONTRIBUTING.md

@@ -35,3 +35,23 @@ Some things that will increase the chance that your pull request is accepted:
 * Write a [good commit message](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html).
 
 *Adapted from [factory_bot_rails's CONTRIBUTING.md](https://github.com/thoughtbot/factory_bot_rails/blob/master/CONTRIBUTING.md).*
+
+## Docker
+
+If you'd rather use a docker container to run the tests, you can use the following instructions.
+
+To set up the container image:
+
+`docker-compose build --pull`
+
+To run specs:
+
+`docker-compose run --rm restforce rspec`
+
+To run rubocop:
+
+`docker-compose run --rm restforce rubocop`
+
+To reset the bundler cache:
+
+`docker-compose down -v`

Dockerfile

@@ -0,0 +1,31 @@
+FROM ruby:2.6.5-alpine
+
+RUN apk add --no-cache \
+  ca-certificates \
+  wget \
+  openssl \ 
+  bash \
+  build-base \
+  git \
+  sqlite-dev \
+  tzdata \
+  tini
+
+ENV LANG en_US.UTF-8
+ENV LANGUAGE en_US:en
+ENV LC_ALL en_US.UTF-8
+
+ENV BUNDLER_VERSION 2.1.4
+RUN gem install bundler -v ${BUNDLER_VERSION} -i /usr/local/lib/ruby/gems/$(ls /usr/local/lib/ruby/gems) --force
+
+WORKDIR /srv
+
+COPY Gemfile restforce.gemspec /srv/
+COPY lib/restforce/version.rb /srv/lib/restforce/version.rb
+
+RUN bundle install
+
+COPY . /srv/
+
+ENTRYPOINT ["/sbin/tini", "-g", "--", "bundle", "exec"]
+CMD ["rspec"]

docker-compose.yml

@@ -0,0 +1,7 @@
+version: "3.6"
+services:
+  restforce:
+    build: .
+    image: restforce:dev
+    volumes:
+      - .:/srv