Introduce a base docker image for layer caching purposes

This will drastically improve docker dev environment initial setup
speed. Rather than building locally, the local docker build (built from
Dockerfile.dev) is now based on the public, automated
exercism/exercism.io build (built from Dockerfile). With this as a
starting point, a developer's `docker-compose build` need only install
any new gems that are introduced in the feature branch. With no local
changes, a local docker build, excluding download time, now takes
approximately 6 seconds, vs. 50 minutes before.

Changes:

- .dockerignore vendor/

    Vendored gems make the docker context directory size large
    (+1.1 G in vendor/), causing longer build times. Additionally, as
    individual developers, rather than being able to inherit most or all
    gems from an application base image, we are forced to install the
    full set of gems when first setting up a development environment.

- Dockerfile is now used by the automated build to build the
  exercism/exercism.io base image. See
  https://hub.docker.com/r/exercism/exercism.io/

- docker-compose builds Dockerfile.dev, which is built FROM the public
  exercism/exercism.io docker image. This Dockerfile need only contain
  commands specific to the development environment and changes from the
  local git repository that need to be built. Presently, it checks for
  new gems to install.

- Dockerfile: Set WORKDIR before RUN bundle install to invalidate one
  fewer layer when the bundle is updated.

.dockerignore

@@ -0,0 +1,7 @@
+.bundle
+.env
+.git
+.gitignore
+log
+tmp
+vendor

Dockerfile

@@ -4,8 +4,7 @@ RUN curl -sL https://deb.nodesource.com/setup_4.x | bash - && \
   apt-get install -y nodejs postgresql-client && \
   npm install -g lineman
 
-COPY Gemfile Gemfile.lock /exercism/
-
 WORKDIR /exercism
 
+COPY Gemfile Gemfile.lock /exercism/
 RUN bundle install

Dockerfile.dev

@@ -0,0 +1,4 @@
+FROM exercism/exercism.io
+
+COPY Gemfile Gemfile.lock /exercism/
+RUN bundle check || bundle install

docker/common.yml

@@ -1,7 +1,9 @@
 version: '2.1'
 services:
   app:
-    build: ..
+    build:
+      context: ..
+      dockerfile: Dockerfile.dev
     env_file: ../.env
     entrypoint: ruby /exercism/docker/entrypoint.rb
     volumes: