Provide a simple way to change the `redmine` version

[cngJo]

This would be great to have a fast way to test a plugin in multiple redmine versions

The following would need to happen to change the version:

1. Update the image tag in the Dockerfile
2. Rebuild the local docker images
3. Start the new redmine image
4. Restore the database

We can pack all of this into a script and pass the version as a parameter.

Another approach would be to have multiple Dockerfiles - one for each version - and start them, depending on a e.G. env variable set in bin/start