add Docker development setup

.docker/ruby/Dockerfile

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+FROM ruby:2.6
+
+RUN apt-get update -qq && apt-get install -y build-essential
+
+# for postgres
+RUN apt-get install -y libpq-dev
+
+# for nokogiri
+RUN apt-get install -y libxml2-dev libxslt1-dev
+
+# for images
+RUN apt-get install -y libmagickwand-dev imagemagick
+
+# for a JS runtime
+RUN curl -sL https://deb.nodesource.com/setup_6.x | bash -
+RUN apt-get install -y nodejs
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends \
+        postgresql-client \
+    && rm -rf /var/lib/apt/lists/*
+
+ENV APP_HOME /app
+RUN mkdir $APP_HOME
+WORKDIR $APP_HOME
+
+ADD Gemfile* $APP_HOME/
+RUN bundle install
+
+EXPOSE 3000

docker-compose.yml

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+version: "3.7"
+
+services:
+  web:
+    build:       
+      context: .      
+      dockerfile: .docker/ruby/Dockerfile
+    command:
+      ruby bin/rails server --port 3000 --binding 0.0.0.0
+    links:
+      - postgres
+      - redis
+    volumes:
+      - ./:/app
+    ports:
+      - 3000:3000
+
+  redis:
+    image: redis:3.2.11-alpine
+    ports: 
+      - 6379:6379
+
+  postgres:
+    image: postgres:10.12
+    ports:
+      - 5432:5432
+    environment:
+      POSTGRES_PASSWORD: justask
+      POSTGRES_DB: justask_development