Auto-Reloading Sinatra Development in Docker

So I've been investigating Docker recently, and its pretty great.

One cool feature which has been particularly useful for me is mounting a directory on the host machine inside the docker container. It is great for a dev environment to auto-reload new changes without building a new container or restarting a container. If you are running an app which auto-reloads you can run a file in a mounted folder, and edit the files locally (on the docker host) and see them reflected in the running containter. Very handy for scripting languages (like Ruby or PHP) as well as cross platform languages (like Java).

I'll go through how I have been doing it for a Sinatra app running with Shotgun (auto-reloader for rackup).

First you need an image with both Sinatra and Shotgun. I pushed one here, but if you prefer to make your own it was just this:

docker run training/sinatra run gem install shotgun 

Now to run it so it auto-reloads from your local folder run a command like this:

docker run -d -p {port-on-host}:9393 -v {absolute-local-path}:/app icchan/shotgun-sinatra shotgun --host 0.0.0.0 /app/{your-app}.rb 

So heres a breakdown:

• -d means run as daemon
• -p {host-port-to-expose}:9393 exposes and publishes port 9393 (shotgun default port) to your host
• -v {absolute-local-path}:/app mounts the local folder to /app in the container
• icchan/shotgun-sinatra is the name of the shotgun image
• shotgun --host 0.0.0.0 /app/{your-app}.rb is the command to run on the image, the 0.0.0.0 is to make accessible outside the container (default is 127.0.0.1)

So if your app is in /home/ruby/app/hello.rb on your host and you want to run it on 8080 you would run this command:

docker run -d -p 8080:9393 -v /home/ruby/app:/app icchan/shotgun-sinatra shotgun --host 0.0.0.0 /app/hello.rb 

It will now be accessible at port 8080 on your host. If you are using boot2docker its probably http://192.168.59.103:8080.

Now edit the file /home/ruby/app/hello.rb on your host machine, the changes will automagically be reflected in the running app :)