Programming exercises: Make docker network customizable

[chrisknedl]

Is your feature request related to a problem?

Customizable docker container CPU, memory and memory-swap flags are currently being added to Artemis. This is a huge improvement, and the ability to define upper bounds in the config makes it more safe.

However, the network flag is currently not as customizable: Containers either use the standard docker network or not use one at all. A custom network can theoretically be set in the config, but that network is shared between all containers created by this Artemis instance.

Describe the solution you'd like

Similarly to the other flags, a list of allowed network names are defined in the config file. During exercise creation, the network can be chosen using a dropdown in the build plan customization section. This gets rid of possible typos that could arise if we'd let developers enter the network name into an input field.

Describe alternatives you've considered

Multiple Artemis instances, each with different network flags, where each instance hosts the exercises that use the corresponding network. This is impractical at best.

Additional context

No response