Docker image for a reproducible environment

[mortenpi]

We should create and publish a standard Docker-based environment in which the users can execute their documentation builds. This would hopefully mean that any issues that DocumentationGenerator encounters could easily be reproduced by anyone. It could also perhaps be reused in package CI.

One small question mark is whether this should be a Docker image which wraps around DocumentationGenerator, or a Docker image that DocumentationGenerator calls. The latter would mean that there would be a Julia interface for executing the Docker image, but would add a Docker dependency to the package (probably would cause issues if you want to run it on Windows for example; it could be opt-in though). But if it's opt-in anyway, it might be easier to just go with the former option, where the Docker image is just a command line interface to the package. The package could also be called in other environments, but for reproducibility it would be best to call the package with the standard Docker image.