Prefer local over remote Docker images

[MoritzWeber0]

When invoking a native Capella installation via Docker, capellambse did always pull the image from a remote environment.

This has two disadvantages:

• It's not possible to work with local images which don't exist in a remote registry.
• It's vulnarable to dependency confusion since attackers could upload a vulnarable Docker image with the same name as the local image to DockerHub.

The new behviour is: capellambse checks if the image exists locally. If it exists, continue with the local image. Otherwise, pull it from the remote.

This also matches the behaviour when using the docker run CLI.