docker: speed up building, improve documentation

Dockerfile

@@ -1,15 +1,6 @@
 # Use an official Ruby runtime as a parent image
 FROM ruby:2.6.4
 
-# Set the working directory in the container
-WORKDIR /usr/src/app
-
-# Clone the bitcoinops.github.io repository
-RUN git clone https://github.com/bitcoinops/bitcoinops.github.io.git
-
-# Change to the repository directory
-WORKDIR /usr/src/app/bitcoinops.github.io
-
 # Install program to configure locales
 RUN apt-get update
 RUN apt-get install -y locales
@@ -26,6 +17,12 @@ ENV LC_ALL C.UTF-8
 ENV LANG en_US.UTF-8
 ENV LANGUAGE en_US.UTF-8
 
+# Set the working directory
+WORKDIR /usr/src/app
+
+# Copy just the Gemfile and Gemfile.lock first to leverage Docker cache
+COPY Gemfile Gemfile.lock ./
+
 # Install any needed gems specified in Gemfile
 RUN bundle install
 

docker-compose.yml

@@ -7,5 +7,4 @@ services:
       - "4000:4000"
     volumes:
       - .:/usr/src/app
-    working_dir: /usr/src/app
     command: make preview