Add dockerignore file

Note there's a difference between gitignore and dockerignore; in
.gitignore, files without a leading slash match files in every folder,
while in .dockerignore, they match files in the root folder

.dockerignore

@@ -0,0 +1,36 @@
+# Version control
+.git
+.gitignore
+
+# Confidential information
+config/database.yml
+config/secrets.yml
+config/deploy-secrets.yml
+config/maintenance.yml
+
+# Logfiles and tempfiles.
+**/log/*
+!**/log/.keep
+**/tmp/
+
+# Files attached by users
+public/system/
+public/ckeditor_assets/
+storage/
+
+# Files generated by scripts or compiled
+public/sitemap.xml
+public/assets/
+public/machine_learning/data/
+
+# Bundler config, cache and gemsets
+**/.bundle/
+**/.ruby-gemset
+
+# Files used in local development
+**/.byebug_history
+**/.DS_Store
+
+# Test results
+spec/examples.txt
+coverage/