add rake docker for consistent build with Docker

Dockerfile

@@ -22,7 +22,3 @@ WORKDIR /opt
 COPY Gemfile .
 COPY Gemfile.lock .
 RUN bundler install
-# RUN vncserver -SecurityTypes None \
-#  && export DISPLAY=$(ls -t ~/.vnc/*.pid | grep -oPm1 ':\d+') \
-#  && bundler exec rake clobber default
-# COPY /opt .

README.md

@@ -250,20 +250,28 @@ fonts when run inside a shell that is attached to [the URxvt terminal][4].
 Building
 --------
 
+### Docker method
+
+If you have Docker available, use the convenient `rake docker` command:
+
+    rake docker
+
+### Manual method
+
 You can build the Tamzen fonts for yourself by running these commands:
 
     bundle exec rake                 # build things only when necessary
     bundle exec rake clobber         # destroy everything that we built
     bundle exec rake clobber default # rebuild everything from scratch
 
-### Requirements
+But first, you'll need to install these dependencies (assuming Debian):
 
     # NOTE: the "xfonts-utils" package provides both bdftopcf(1) and fonttosfnt(1)
     sudo apt-get install ruby git imagemagick xfonts-utils bdf2psf default-jre gbdfed
     gem install bundler
     bundle install
 
-Requirements for [bitmap-font-patcher][6]:
+Similarly, if you've opted to use [bitmap-font-patcher][6] integration:
 
     sudo apt-get install python python-fontforge python-pip python-dev libfreetype6-dev
     pip install --user bdflib

Rakefile

@@ -1,4 +1,5 @@
 require 'tempfile'
+require 'shellwords'
 require 'rake/clean'
 
 task 'default' => %w[
@@ -10,6 +11,51 @@ task 'default' => %w[
   .screenshots
 ]
 
+#-----------------------------------------------------------------------------
+# docker
+#-----------------------------------------------------------------------------
+
+DOCKER_IMAGE = 'tamzen-font'
+
+task :docker => 'docker:container'
+
+task 'docker:image' => %w[ Dockerfile Gemfile Gemfile.lock ] do |t|
+  sh "tar -cf- #{t.prerequisites.shelljoin} | docker build -t #{DOCKER_IMAGE} -"
+end
+
+task 'docker:container' => 'docker:image' do
+  container = `
+    docker create \
+      -v #{Dir.pwd.shellescape}:/run/input #{DOCKER_IMAGE} \
+    sleep infinity
+  `.chomp
+  raise unless $?.success?
+
+  begin
+    sh "docker start #{container}"
+    sh "docker exec #{container} rake -f /run/input/Rakefile docker:exec"
+    sh "docker cp #{container}:/opt _build"
+  ensure
+    sh "docker rm -f #{container}"
+  end
+
+  sh "rsync -auv _build/ ./ --exclude=_build"
+end
+CLOBBER.include %w[ _build ]
+
+task 'docker:exec' do
+  raise unless Dir.pwd == '/opt'
+  sh "cp -pRT /run/input ."
+
+  ENV['DISPLAY'] = ':1'
+  sh "vncserver -SecurityTypes None $DISPLAY"
+  begin
+    sh "bundle exec rake clobber default"
+  ensure
+    system "vncserver -kill $DISPLAY" # exits nonzero
+  end
+end
+
 #-----------------------------------------------------------------------------
 # bitmap
 #-----------------------------------------------------------------------------