Update bundler & ensure GEM_HOME is set in container

Also, add docker-compose.yml for local development & update README for
that workflow.

Dockerfile

@@ -3,17 +3,16 @@ ARG UNAME=app
 ARG UID=1000
 ARG GID=1000
 ARG APP_HOME=/app
-ARG GEM_HOME=/gems
 
+RUN gem install 'bundler:~>2.1.4'
 RUN groupadd -g $GID -o $UNAME
 RUN useradd -m -d $APP_HOME -u $UID -g $GID -o -s /bin/bash $UNAME
-RUN mkdir -p $GEM_HOME && chown $UID:$GID $GEM_HOME
+RUN mkdir -p /gems && chown $UID:$GID /gems
 
 USER $UNAME
-RUN gem install 'bundler:~>1.17.3' 'bundler:~>2.0.2'
 COPY --chown=$UID:$GID Gemfile* ${APP_HOME}/
-ENV BUNDLE_GEMFILE=${APP_HOME}/Gemfile
-ENV BUNDLE_PATH=${GEM_HOME}
+ENV BUNDLE_GEMFILE ${APP_HOME}/Gemfile
+ENV BUNDLE_PATH /gems
 WORKDIR ${APP_HOME}
 
 RUN bundle install

README.md

@@ -9,6 +9,13 @@ Puppet Nebula
 # Development
 
 1. git clone
-2. bundle
-3. `(bundle exec) rake spec_prep`
-4. `(bundle exec) rspec # or rake spec_standalone`
+2. `docker-compose build`
+3. `docker-compose run spec_prep`
+4. `docker-compose run specs`
+
+or:
+
+```bash
+docker-compose run specs bundle exec rspec specs/path/to/a_spec.rb
+docker-compose run specs bundle exec rake spec_standalone
+```

docker-compose.yml

@@ -0,0 +1,22 @@
+version: '3'
+services:
+  specs:
+    build: .
+    command: 'bundle exec rake spec'
+    volumes:
+      - .:/app
+
+  spec_prep:
+    build: .
+    command: 'bundle exec rake spec_prep'
+    volumes:
+      - .:/app
+
+  lint:
+    build: .
+    command:
+    - /bin/bash
+    - '-c'
+    - 'bundle exec rake rubocop && bundle exec rake syntax lint && bundle exec rake metadata_lint'
+    volumes:
+      - .:/app