Add --pull to docker build

Add --pull to docker build

diff --git a/Rakefile b/Rakefile
index 70bcc22..095586b 100644
--- a/Rakefile
+++ b/Rakefile
@@ -36,8 +36,7 @@ end
 namespace :docker do
   desc "Build a new docker image"
   task :build do
-    sh "docker pull ruby:2.6-alpine"
-    sh "docker build -t discourse/ddns-sd --build-arg=http_proxy=#{ENV['http_proxy']} --build-arg=GIT_REVISION=$(git rev-parse HEAD) ."
+    sh "docker build --pull -t discourse/ddns-sd --build-arg=http_proxy=#{ENV['http_proxy']} --build-arg=GIT_REVISION=$(git rev-parse HEAD) ."
   end
 
   desc "Publish a new docker image"

GitHub sha: c1e8f7fedae0011bf8ded0cabab4b11ed0209e65