I’ve decided to add my own answer with the best solution I’ve found so far. I’ll update this if I find better options.

Best solution so far

The best solution I’ve found for setting up a productive development environment with Docker on OS X is: Boot2Docker + Rsync. With rsync, build times in a Docker container are on par with running the build directly on OSX! Moreover, the file watcher code does not need polling (inotify works since rsync uses normal folders), so hot reload is almost as fast.

There are two ways to set it up: an automated install and a manual install.

Automated install

I’ve packaged all the steps for setting up Boot2Docker with Rsync into an open source project called docker-osx-dev. The code is a bit rough, but I’ve been successfully using it for several weeks to easily switch between 3 projects with 3 different tech stacks. Try it out, report bugs, and submit some PRs! Also, see my blog post, A productive development environment with Docker on OS X for more info.

Manual setup

1. Install Boot2Docker: brew install boot2docker.
2. Run Boot2Docker, but with VirtualBox shared folders disabled: boot2docker init && boot2docker start --vbox-share=disable.
3. Run boot2docker shellinit and copy the environment variables it prints out into your ~/.bash_profile file.
4. Install rsync on the Boot2Docker VM: boot2docker ssh "tce-load -wi rsync".
5. Create the base folders you need on the Boot2Docker VM and set permissions correctly for them. For example, if you’ll be syncing the /foo/bar folder from OS X, you need to create /foo/bar on the Boot2Docker VM: boot2docker ssh "mkdir -p /foo/bar && chown -R docker /foo/bar".
6. Run rsync to sync the files to the Boot2Docker VM: rsync --archive --rsh="ssh -i $HOME/.ssh/id_boot2docker -o StrictHostKeyChecking=no" /foo/bar docker@dockerhost:/foo. Check the rsync docs for various settings you may want to enable, such as using --exclude .git to exclude the .git folder when syncing.
7. Use a file watcher to keep files in sync. For example, you could use fswatch (brew install fswatch) piped into rsync.
8. At this point, you should be able to use docker run to fire up your Docker container and use the -v flag to mount the folder you’re syncing: docker run -v /foo/bar:/src some-docker-image.
9. Update the code on OS X as usual. Changes should propagate very quickly using rsync, the normal file watcher code should pick up the changes as usual (ie, using inotify), and the build should run fast because all the files are “local” to the container.
10. If you need to test a running website, run the boot2docker ip command to find out what IP it’s on.