Create test task for Docker compose (swiftlang#78)

Author: shahmishal

docker-compose.yaml

@@ -38,6 +38,13 @@ services:
         && bundle exec jekyll doctor --source /srv/jekyll --destination /output
         && bundle exec jekyll build --source /srv/jekyll --destination /output"
 
+   test:
+      <<: *common
+      command: /bin/bash -cl "
+        bundle check
+        && bundle exec jekyll doctor --source /srv/jekyll --destination /output
+        && bundle exec jekyll build --source /srv/jekyll --destination /output"
+
    website:
      <<: *common
      command: /bin/bash -cl "bundle check && bundle exec jekyll serve --source /srv/jekyll --destination /output --host 0.0.0.0 --watch"