Our Docker image uses Chromium v57, which is not supported by
chromedriver versions after 2.38. See
https://bugs.chromium.org/p/chromedriver/issues/detail?id=2445
The reason that the version of chromium is so old is that our base
image ruby:2.3.6 (the standard image for Ruby 2.3.6 on DockerHub) is
built on Debian 8 (jessie), which was made obsolete over a year ago.
The best solution is probably to upgrade the application to Ruby 2.3.7
(if not later), so we can use the official ruby:2.3.7 image, which is
kept up-to-date (it has Chromium 68).
Alternatively, we could try to install Chromium from a more up-to-date
repository, but this is probably not worth the trouble.
The following enhancements have been made to docker/docker-compose
* Fixed bug when building the image.
* docker-compose up starts the server
* Scaffolding inside the container respect the ownership of the files
outside it
* Volumes are tagged as 'delegated' in order to improve performance for
mac/windoze users.
* bundler stores packages in a volume. This whay new packages can be
added without rebuilding the image:
```bash
docker-compose run app bundle install
```
