clean up the c# image

[bmesuere]

The C# image doesn't adhere to docker file best practices and doesn't pass our linting. While this might look innocent, it can have unexpected consequences: images built at a different time on a different worker node can behave differently.

To fix this, the following changes need to be made:

dodona-csharp.dockerfile:1 DL3006 Always tag the version of an image explicitly
dodona-csharp.dockerfile:4 DL3009 Delete the apt-get lists after installing something
dodona-csharp.dockerfile:5 DL3008 Pin versions in apt get install. Instead of `apt-get install <package>` use `apt-get install <package>=<version>`
dodona-csharp.dockerfile:5 DL3015 Avoid additional packages by specifying `--no-install-recommends`
dodona-csharp.dockerfile:6 DL3008 Pin versions in apt get install. Instead of `apt-get install <package>` use `apt-get install <package>=<version>`
dodona-csharp.dockerfile:6 DL3015 Avoid additional packages by specifying `--no-install-recommends`
dodona-csharp.dockerfile:19 DL3002 Last USER should not be root

Inspiration on fixing these can be found at #13 and at https://github.com/hadolint/hadolint/wiki.