To avoid reinstalling dependencies you can use multiple dependency files to separate your slowest building dependencies (i.e for ruby you can use a Gemfile and Gemfile.tip) and a git hook script to set the modified time of all files in the repo to their last change in git:
We looked at the approach of adding a Gemfile.tip, however we like the data volume approach because it doesn't require any changes to the Dockerfile and it is more like what our developers are familiar with. A Gemfile.tip can become basically another Gemfile eventually, so separating the bundle step from the build step (in Development) gives us more flexibility as well as keeping things more in line with what our developers expect to do.
What does setting the modified time of all files do?
The biggest pain I've run into when using Docker for local dev is waiting for pip to install dependencies on rebuilds. This offers an interesting strategy for mitigating that and I look forward to digging into this more.
We experienced that exact same pain. Whether it's pip or Bundler, I can't tell you how many times I've installed and reinstalled requirements.
Sharing data volumes is kind of a hack to make it really easy to keep part of a container around when you delete and recreate a container. I would love to see persistent data volumes become first class citizens so you don't have to create a separate container for them.
For now, though, it's saved us from having to put everything directly into the image in development.
I wrote djtempl ( https://github.com/emailgregn/djtempl ) for my purposes.
Link the to github project: https://github.com/IFTTT/dash
