Why should I use a Dockerfile instead of a batch script?

Question

I'm new to Docker and feeling my way around. My plan is to build a typical web app using Nginx+Rails+Postgress, all of which will be in the one container. I'm not (currently) doing anything complex like linking images.

I'm a lone developer, and my build process thus far is:

1. Edit Dockerfile
2. docker build
3. Fix bugs, and if I like the outcome then commit the Dockerfile to a git repo.
4. Iterate over steps 1-3 as I change the build.
5. docker push my/image periodically, as useful versions emerge.

Why instead would I not:

1. docker pull a basic image e.g. Ubuntu
2. docker run -t -i my/image /bin/bash
3. wget http://git.host.com/installation-script.sh | bash
4. If bugs then scrap image and edit installation-script.sh to fix bugs.
5. Iterate over 1-4.
6. docker push my/image periodically, as above.

I'm aware of issues with 'wget shell-script | bash', however it would be more familiar to me.

Instinctively I feel that using a Dockerfile is the best way to go, but I'm not sure why. I think it would be useful for Docker beginners to understand why Dockerfile is (or isn't) best practice. If I was deploying linked containers would I realise the awesome power of the Dockerfile? Does Dockerfile affect the "quality" (size, whatever?) of the final image?

Answer 1

One great reason for doing as much work as you can in the Dockerfile is Docker caching. A great practical example of this is for skipping bundle install on rails apps every time you test a new build.

You might be able to emulate caching using your script combined with a docker volume, but you'd be stuck doing a lot of your own plumbing work.